Experimental Investigation of a Solar Dryer System for Drying Carpet

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Energy Procedia 70 ( 2015 ) 626 – 633

ScienceDirect

Peer-review by the scientific conference committee of SHC 2014 under responsibility of PSE AG doi: 10.1016/j.egypro.2015.02.170

International Conference on Solar Heating and Cooling for Buildings and Industry, SHC 2014

Experimental investigation of a solar dryer system for drying carpet

Guofeng Yuan

a*

, Liang Hong

b

, Xing Li

a

, Li Xu

a

, Wenxue Tang

c

, Zhifeng Wang

a

a_{Key Laboratory of Solar Thermal Energy and Photovoltaic System of Chinese Academy of Sciences, Institute of Electrical Engineering, Beijing}

100190, China

b_{Energy Research and Demonstration Center of Tibet Autonomous Region, Lhasa 850001,China} c_{Guangdong 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/(m2_{·k) 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_.

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

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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 heaterbecome 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

F

R

(

WD

)

and

F

R

U

L under steady state condition [21].

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 m}2_{, distribution tube number:10, distribution tube diameter: 50 mm.}

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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,

G

t

U

m a G G G

0

K

(1) Where,

K

_G is the instantaneous efficiency,

K

₀_Gis the efficiency when the reduced temperature difference (

G

t

m

a _{) equal to zero (Cut Length of efficiency),}

G

U

is the overall heat loss coefficient base on the gross area (W/(m2_{· K)),}

m

t

is the mean temperature of heat transfer fluid (°C),

t

_ais 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 – 850 ±2ˁ

Flow gauge (AKTUX-80) 28 – 400 m3_{/ h} _±1.5_ˁ

Pyranometer (WXL20-LVRZC-12) 0-2000W m-2_±2_ˁ

Differential pressure gauge (DPG2k) 0-2000 Pa ±3ˁ

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Base on the ASHRAE Standard 93-2010, the test conditions as follows: solar irradiance is greater than 800 W/m2_{, and vary not more than ±32 W/m}2_{, average wind velocity shall be between 2.0 m/s and 4.0 m/s, ambient} temperatures less than 30ć, the air flow rate is 165 m3_/h.

0.02 0.03 0.04 0.05 0.06 0.40 0.45 0.50 0.55 0.60 0.65 0.70 in stantaneo us eff ici enc y (t_m-t_a)/G 40 60 80 100 120 140 160 180 0 100 200 300 400 500 R es is ta nce(Pa) 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.

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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

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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]:

AI

T

C

m

_P _o _i f

)

(

K

(2)

In which,

m

is the m specific heat capacity rate of the air flow,

C

_P is the specific heat capacity at constant pressure,

T

_o and

T

_i 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]:

d d r

W

m

0

0 (3)

In which,

m

_r₀is the initial moisture ratio,

W

₀ is the initial weight of the wet products,

W

_d is the basic weight of dry products.

The instantaneous moisture content ratio on dry basis at any time is expressed as the following equation:

d d t r

W

m

(4) In which,

m

_ris 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 100ć 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 m}3_{/h, and} the air density is 0.81 kg/m3 _in_{Lhasa region.}

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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 80ć, when the solar irradiation reached 1100 W/m2_{, and the average temperature of the dryer} chamber (Ttank) reach to 80ć. 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 70~80ć,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 80ć and 60ć respectively, and there are about 450 minutes when the moisture content ratio decrease to 0.01.

09:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 0 20 40 60 80 100 120 140 160 Tem perat ure( ć ) Time (hh:mm) Tinlet Tout Ttank 200 400 600 800 1000 1200

Solar irradiation Solar irradiat

ion(W /m 2) 09:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12 20 40 60 80 100 120 140 160 Te mpe rat ur e( ć ) Time (hh:mm) Tinlet Touelet Ttank 0 200 400 600 800 1000 1200 Solar irradition S olar irr ad iatio n( W /m 2)

Fig.8 Solar irradiation and air temperature variation for test1 Fig.9 Solar irradiation and air temperature variation for test2

09:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12 20 30 40 50 60 70 80 90 100 Tem perat ure( ć ) Time (hh:mm) Tinlet Tout Ttank 0 200 400 600 800 1000 1200 Irradiation S o lar irrad iation (W /m 2) 0 100 200 300 400 500 0.0 0.2 0.4 0.6 0.8 1.0 M ois ture rat io (%)

Drying time (min)

test1 test2 test3

Fig.10 Solar irradiation and air temperature variation for test3 Fig.11 Moisture ratio variation at different test conditions

4.Conclusions

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

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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).

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