CC BY-NC-ND
0<S)
CrossMark
Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 70 (2015) 626 - 633
International Conference on Solar Heating and Cooling for Buildings and Industry, SHC 2014
Experimental investigation of a solar dryer system for drying carpet
Guofeng Yuana*, Liang Hongb, Xing Lia, Li Xua, Wenxue Tangc, Zhifeng Wanga
aKey Laboratory of Solar Thermal Energy and Photovoltaic System of Chinese Academy of Sciences, Institute of Electrical Engineering, Beijing
100190, China
bEnergy Research and Demonstration Center of Tibet Autonomous Region, Lhasa 850001,China cGuangdong Fivestar Solar Energy Co., Ltd, Dongguan 523051, China
Abstract
The work presents a solar dryer system for carpet drying. A flat plate solar air heater was designed for the large scale industry drying process. The thermal performance of the designed collector was tested based on the ANSI/ASHRAE Standard 93-2010, and the test results show that the instantaneous efficiency and heat loss coefficient base on gross area and mean temperature of heat transfer fluid is 0.77 and 5.62W/(m2k) respectively. Then, a solar dryer system was designed and constructed for carpet drying, which composed of a 56 m2 solar air heater field, a dryer cabinet and others subsystems. Performance of the drying system was tested and analyzed, and the test results show that more than 320kg wet wool and carpet could be dried in 7.5 hours, when the average intensity of solar irradiation got to 800 W/m2.
© 2015TheAuthors.Published by ElsevierLtd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review by the scientific conference committee of SHC 2014 under responsibility of PSE AG Keywords: Solar air heater; Solar dryer; Heat loss coefficient; Resistance
1. Introduction
The use of solar energy in drying is becoming an important and viable alternative since it decreases consumption of conventional energy, and improves production efficiency. Solar drying helps to overcome the inherent disadvantage of open sun drying i.e. it protects the product from unpredicted rain, wind-borne dirt and dust, infestation by insects, rodents etc.. [1,2]. Using a solar dryer, the drying time can be shortened by about 65% compared to the open sun drying because the efficiency of dryer is higher, and its payback period ranges from 2~4 years depending on the rate of utilization [3].
* Corresponding author. Tel.: +86-10-62527385; fax: +86-10-62587946. E-mail address: yuanguofeng@163.com
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review by the scientific conference committee of SHC 2014 under responsibility of PSE AG doi:10.1016/j.egypro.2015.02.170
In recent past, various solar dryers have been developed for the efficient utilization of solar energy. Many studied have been reported on solar drying of agricultural products [4-6]. Solar drying systems are classified primarily according to their heating modes and the manner in which the solar heat is utilized. In broad terms, they can be classified into two major groups, namely active solar energy drying systems and passive solar energy drying systems. Passive solar energy drying systems conventionally termed natural circulation solar drying systems, which do not require fans to pump the air through the dryer, and they are low-cost, can be locally constructed and do not require energy from fossil fuels. However, some drying limitations because of lower efficiency in comparison with forced convection ones may be expected [7,8]. A typical active solar dryer depends solely on solar energy as the heat source but employs motorized fans and/or pumps for forced circulation of the hot air through the drying chamber. The forced air circulation will get a higher efficiency compared to the natural cycle one, and the electric power requirement of the fan is very low and can be operated by one photovoltaic module independent of the electric grid [9-11].Solar air heater is the basic component of the active solar dryer, and the performance of solar drying system strongly depends on the performance of solar air heater [12], and there are several type of solar air heater had used in the solar dryers in the past researches [13-20].
The main objective of this study was to design an active solar dryer for a carpet factory. Base on reviews and experience, a solar air heater was designed for the drying system firstly, and then, a solar drying system was designed and tested for carpet and wool drying. The following introduce the configurations and performance test results of this research.
2. Performance testing of solar air heater
2.1. Description of the solar air heater
Solar air heater is the basic component of solar dryer, and the performance of solar air heater become imperative to test and compare on equitable basis for the solar dryer system design. For solar water and air heaters, ANSI/ASHRAE Standard 93-2010 recommended the instantaneous efficiency test for measuring thermal performance characterized by FR (ra) and FRUL under steady state condition [21].
Aluminum fins
= -=/- = - =
Insulation Selective Absorber
Fig. 1 Schematic view of the designed solar air heater
The collector under consideration is a flat plate solar air collector which consisting of a transparent cover, a blackened metal sheet (absorber) and an insulated base that delimitates an air duct. The insulation at the front consists of a layer of air trapped between the transparent cover and the absorber. The straight aluminum fins were weld in the air duct to strengthen the heat transfer efficiency between absorber. As shown in the fig.1, the staggered arrangement of the fins was used to improve the uniformity of the air flow distribution in the flow channels. The geometric characteristics of the solar air heater as follows: width: 1000mm, length: 2000mm, fin spacing: 15mm, total gross area: 2m2, absorber area: 1.96 m2, distribution tube number:10, distribution tube diameter: 50 mm.
2.2. Thermal performance tests
The performance of the designed solar air heater was tested base on the ASHRAE Standard 93-2010 [21]. The thermal performance of the solar air heaters were expressed as the relationship between instantaneous efficiency and reduced temperature difference, as the following function,
Vg =VoG - U-Jm-JL G
Where, ?7G is the instantaneous efficiency, r/0G is the efficiency when the reduced temperature difference ' m ) equal to zero (Cut Length of efficiency), UG is the overall heat loss coefficient base on the gross area
(W/(m2- K)), tm is the mean temperature of heat transfer fluid (°C), ta is the Ambient air temperature (°C), G is the solar irradiation (W/m2).
Fig. 2. The solar air heater performance test system
As shown in fig.2, an open-loop system base on induced draft system was used, in which, the blower is located in the downstream of the collector in order to produce a negative gauge pressure in the collector. The differential pressure gauge, air flow rate meter and temperature sensors were used for the performance testing. Two flow meters were installed in the upstream and downstream air tube of the tested collector respectively. The Pt100 thermocouple was used to measure the temperatures of the inlet and outlet of the solar air heater, and there are five thermocouples located at the center of equal cross-sectional areas for the air temperature test in every test point. The differential pressure gauge was used for the pressure drop test in the solar air heater. There are three group of electric heater was used for create the different mean temperature of heat transfer fluid.
Table 1. An example of a table.
Name of the apparatus Measuring range Accuracy
Thermocouple (Pt100) -200 - 850D ±2%
Flow gauge (AKTUX-80) 28 - 400 m3/ h ±1.5%
Pyranometer (WXL20-LVRZC-12) 0-2000W m-2 ±2%
Differential pressure gauge (DPG2k) 0-2000 Pa ±3%
Data acquisition instrument (Agilent 4970A) ---- ----
Base on the ASHRAE Standard 93-2010, the test conditions as follows: solar irradiance is greater than 800
temperatures less than 30 °C, the air flow rate is 165 m3/h.
W/m2, and vary not more than ±32 W/m2, average wind velocity shall be between 2.0 m/s and 4.0 m/s, ambient
£ 200
Flow rate(m3/h)
Fig.3 Thermal performance of the designed solar heater
Fig.4 Pressure drop of the designed solar air heater
The instantaneous efficiency and pressure drop character of the designed plate solar air heater was test, as shown in fig.3 and fig.4. The test results show tha the instantaneous efficiency reach to 0.773 when the reduced temperature difference equal to zero, at the same time, the overall heat loss coefficient base on gross area is 5.63 W/(m2 K), and the resistance is 400 Pa, when the air flow rate equal to 165m3/h.
Air inlet solar air heating system
Fig.5 Schematic diagram of solar dryer system
3. Solar dryer system description and tests
3.1. Solar dryer system description
A solar drying system was designed and constructed for Lhasa chengguan district carpet factory. The layout of the drying system as shown in fig.5, which consists of the following subsystem: drying chamber, solar air collector field, solar water field, hot water tank, fan, sensors and other subcomponents. The detail descriptions of the main components are as follows.
3.1.1 Solar air heater field
The drying system employs 28 pieces of plate solar air heaters with a total gross surface area is approximately 56 m2, and the designed plate solar air heater was used in this system. As shown in fig.6, in order to organize the solar air heater field system, the solar air heaters were divided into 7 parallel groups, and each group consists of 4 pieces of solar air heater connected in series. The rectangular structure plug-in connecting method was used between the adjacent collectors to reduce the local resistance. The 7 groups of the collectors share a trunk tube, and the design wind speed in the trunk tube is less than 4m/s.
Fig.6 Photograph of the solar air heater field Fig.7 Photograph of the drying chamber
3.1.2 Solar dryer cabinet
As shown in fig.7, a dryer cabinet was designed and constructed for a production capacity of 150 kg dry blanket and wool per day, and the dimension is 2.0m*3.7 m*1.5m. A circulating fan was used to strengthen the convention intensity and optimized the air flow distribution in the drying chamber. A water heating and water/air heat exchanger system was installed as an auxiliary drying system in the evenings and raining days. A movable drying racks was mounted inside the drying chamber for the knitting wool and carpet arrangement. There is an exhaust fan was used for the moist air emission.
3.1.3 Data acquisition and control system
As shown in fig.5, all important variables were measured and recorded during the experimental tests, and the metrology equipment of the drying system consists of as the following: (1) Platinum resistance thermometers (Pt100, measurement range -40~200°C, accuracy ±0.2°C), four-wire, calibrated for measuring: ambient temperature, solar air field inlet/outlet temperatures, drying chamber temperatures, air flow temperature after the water/air heat exchanger, water/air heat exchanger inlet/outlet water flow temperature; (2) Relative humidity and temperature transmitters (TH200, measurement range -40~180°C, 0~100% RH ; accuracy ±0.2°C,±1.5% RH), for measuring the temperature and relative humidity of the drying chamber; (3) Liquid turbine flower meter (FM110, measurement
range 0.5~10m3/h, accuracy ±0.5%), for measuring the water flow rate in the water/air heat exchanger; (4) Hot-wire anemometer (HR100, measurement range 0~10m/s, accuracy ±0.5%), for measuring the air flow rate of the solar air field; (5) Pyranometer (TBQ-2-B, measurement range 0~2000W/m2, accuracy ±2% ), for measuring the solar irradiation. (6) Digital scales (accuracy ±0.1kg), for measuring the product weight during the drying process. All instrumentations, calibrated in advance to determine their probing sensibilities, were connected to a PLC data acquisition and control system, which driven by a PC.
3.1.4 Material
This system was designed for the carpet drying, which produced by Lhasa Chengguan district carpet factory. The carpet fabricate process is very complex, which including wool washing, twisting, dyeing, cutting and carpet washing etc.. The average moisture content of the wet wool and carpet was found about 100%, and there are about 3 sunny days were needed for the wet wool and carpet drying process by the traditional open sun drying process, and the production capacity was restricted by the slowly drying process.
3.2. System test and analysis
The thermal performance of the solar air heater field was tested according to ASHRAE standard. The fluid inlet and outlet temperatures, fluid flow rate had been measured, and finally the solar field thermal efficiency is calculated using the following equation [22]:
m cp (To - t )
Vf =—^r—- (2)
In which, m is the m specific heat capacity rate of the air flow, CP is the specific heat capacity at constant
pressure, To and Ti is the air flow inlet and outlet temperature of the solar air heater field, A is the area of solar
field, I is the solar incident radiation .
The moisture content on dry basis is the ratio of moisture weight in products per moisture weight of dry products, and the initial moisture contents is calculated as the following equation[23]:
mr 0 = —0—^ (3)
In which, mr0 is the initial moisture ratio, W 0 is the initial weight of the wet products, Wd is the basic weight of dry products.
The instantaneous moisture content ratio on dry basis at any time is expressed as the following equation:
W - wd
mr =-i-d- (4)
In which, mr is the instantaneous moisture content ratio, W t is the instantaneous weight of the drying products.
The solar drying system performance and characteristics have been tested and discussed under three different air flow rate and mass loads conditions.
Fig.8~fig.10 shows the variation of the solar irradiation, inlet and outlet temperature of solar air heater field, dryer chamber temperature during the test days. The outlet temperature depended upon the solar irradiation. It increased in the morning, and reaches peak value in noon and stared decreasing in afternoon. According to Fig.8 and Fig.9, the maximum value of outlet air temperature was higher than 100 °C when the solar irradiation reached 1100 W/m2, and the solar field thermal efficiency is more than 48%, in which, the air flow rate was set to 1500 m3/h, and the air density is
0.81 kg/m3 in Lhasa region.
Fig.8 shows the air temperature and solar irradiation condition of test1, in which, the air flow rate and dry mass load was set to 1500 m3/h and 160kg (320kg wet carpet) ) respectively. As shown in fig.8, the air flow temperature different reach to 80 °C, when the solar irradiation reached 1100 W/m2, and the average temperature of the dryer chamber (Ttank) reach to 80°C. In this test, the moisture ratio decrease to 0.5 in 150 minutes, then, the moisture variation is slowdown, and there are about 360 minutes when the moisture ratio decrease to 0.01, because of the dense weaving of the carpet.
Fig.9 shows the experimental condition of the test2, in which, the dry mass load was set to 200 kg (400kg wet carpet) ). As shown in fig. 9 and fig.11, the temperature of the drying chamber (Ttank) between 70~80°C,and there are still 10% of the moisture in the carpet after 8 hours solar drying, at the same time, part of the carpet is discolored, because of the local high temperature, which caused by the overload and uneven air flow distribution in the drying chamber.
Then, the test3 was carried, in which, the air flow rate and dry mass load was set to 2000 m3/h and 160 kg (320kg wet carpet) ) respectively. As shown in fig.10 and fig.11, the outlet air flow temperature of solar air heater field (Tout) and the average temperature of the drying chamber (Ttank) was decrease to 80 °C and 60 °C respectively, and there are about 450 minutes when the moisture content ratio decrease to 0.01.
p 100-
2Í ■ a 80-
- 800 Ü
p 100.
________
/ / -------Tinlet ^ -
/ Touelet
t Ttank
12:00 13:12 14:24 15:36 Time (hh:mm)
9:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12 Time (hh:mm)
Fig.8 Solar irradiation and air temperature variation for testl
Fig.9 Solar irradiation and air temperature variation for test2
-----Tinlet
Tout Ttank
■ Irradiation
09:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12 Time (hh:mm)
100 200 300 400
Drying time (min)
Fig.10 Solar irradiation and air temperature variation for test3
4. Conclusions
Fig.11 Moisture ratio variation at different test conditions
Ü 50-
In this paper, a solar air heater were tested based on ASHRAE Standard 93-2010, The test results show that, the resistance of the improved solar heater's decrease to 400 Pa. Then, a solar drying system was designed and experimental investigated base on the new designed solar air heater, the test results showed that there are more than
320kg wet wool and carpet could be dried in 7.5 hours in Lhasa Tibet, when the average intensity of solar irradiation got to 800 W/m2, and the average thermal efficiency of the solar air field can reach to 48%.
Acknowledgements
This work has been supported by National Natural Science Foundation of China (No.51106150), National Natural Science Foundation of China (No.51476164), National Science and Technology Infrastructure Program (No.2012BAA05B06), the Introduction of Innovative R&D team Program of Guangdong Province (No.2013N070).
References
[1] Doymaz I. Dryong kenetics of wite mulberry. Journal of food engineering 2004;61(3):341-346.
[2] Pangavhane D.R.,Sawhney R.L.,Sarsavadia P.N.. Development ¿performance testing of a new natural convection solar dryer. Energy 2002; 27(6):579-590.
[3]Sacilik K. Effect of drying methods on thin-layer drying characteristics of the hull-less seed pumpkin. Journal of food engineering 2007;
79(1):23-30.
[4] Leon M.A, Kumar S, Bhattacharya S.C.. A comprehensive procedure for performance evaluation of solar food dryers. Renewable & Sustainable Energy Reviews 2002; 63:67-93.
[5] Ekechukwe O.V., Norton B.. Review of solar energy drying systems II: an overview of solar drying technology. Energy Conversion and Management 1999;40:616-55.
[6] A. Fudholi, K. Sopian. Review of solar dryers for agricultural and marine products. Renewable and Sustainable Energy Reviews 2010; 14:130.
[7] Forson, F.K., Nazha, M.A.A., Rajakaruna, H.. Modelling and experimental studies on a mixed-mode natural convection solar cropdryer. Solar
Energy 2007; 81:346-357.
[8] Jain, D., Tiwari, G.N.. Effect of greenhouse on crop drying under natural and forced convection. I: Evaluation of convective mass transfer coefficient. Energy Conversion and Management 2004; 45: 765-783.
[9] Hossain, M.A., Bala, B.K.. Drying of hot chilli using solar tunnel dryer. Solar Energy 2007;81: 85-92.
[10] Bala, B.K., Ashraf, M.A., Uddin, M.A., Janjai, S.. Experimental and neural network prediction of the performance of a solar tunnel dryer for drying jackfruit bulbs and leather. Journal of Food Process Engineering 2005; 28: 552-566.
[11] Chen, H.H., Hernandez, C.E., Huang, T.Z.. A study of the drying effect on lemon slices using a closed-type solar dryer. Solar Energy 2005; 78:97-103.
[12] Kalogirou SA. Solar thermal collectors and applications. Prog Energy Combust Sci 2004; 30(3):231-95.
[13] Zomorodian A, Zare D, Ghasemkhani H. Optimization and evaluation of a semicontinuous solar dryer for cereals (Rice, etc.). Desalination 2007; 209:129-35.
[14] Yan WJ, Hu YY. Solar air heating for drying. In: Rudolph V, Keey RB, Mujumdar AS, editors. Proceedings of the 9th international drying symposium (IDS 1994), 1-4 August Gold Coast, Australia,1994.
[15] Chr. Lamnatou, E. Papanicolaou. Experimental investigation and thermodynamic performance analysis of a solar dryer using an evacuated-tube air collector. Applied Energy 2012,94:232-243.
[16] Alireza Mohajer, Omid Nematollahi .Experimental investigation of a Hybrid Solar Drier and Water Heater System. Energy Conversion and Management. 2013; 76: 935-944.
[17] Alejandro Reyes, Andrea Mahn. Mushroom dehydration in a hybrid-solar dryer. Energy Conversion and Management 2013;70:31-39
[18]A.A. El-Sebaii, S.M. Shalaby. Experimental investigation of an indirect-mode forced convection solar dryer for drying thymus and mint .Energy Conversion and Management 2013; 74:109-116
[19] Wisut Chramsa-ard, Sirinuch Jindaruksa. Performance evaluation of the desiccant bed solar dryer. Energy Procedia 2013; 34 189-197.
[20] Erick Cesar Lo'pez-Vidan~ a, Lilia L. Me'ndez-Lagunas. Efficiency of a hybrid solar-gas dryer. Solar Energy 2013; 93 :23-31.
[21] ANSI/ASHRAE Standard 93-2010.Methods of Testing to Determine the Thermal Performance of Solar Collectors[S].2010.
[22] John A. Duffie, William A.Beckman. Solar engineering of thermal process. A Wiley-Interscience publication, 1991.
[23] El-Sebaii AA, Aboul-Enein S, Ramadan MRI, El-Gohary HG. Experimental investigation of an indirect type natural convection solar dryer. Energy Convers Manage 2002; 43:2251-66.
