Clean Techn Environ Policy (2011) 13:125–132 DOI 10.1007/s10098-010-0279-3
ORIGINAL PAPER
Design and development of energy efficient solar tunnel dryer for industrial drying N. S. Rathore • N. L. Panwar
Received: 25 September 2009 / Accepted: 20 January 2010 / Published online: 6 February 2010 Ó Springer-Verlag 2010
Abstract A natural convection poly house walk-in type solar tunnel dryer was designed and used for drying surgical cotton on industrial scale. This article deals with the basic design criterion used for development of solar tunnel dryer and result of drying of surgical cotton in actual use. A batch of surgical cotton of 600 kg by mass, having an initial moisture content of 40% wet basis from which 210 kg of water is required to be removed to get a desired moisture content of about 5% wet basis, is used as the drying load in designing the dryer. A drying time of 7–8 h is assumed for the anticipated test location (Udaipur, 27° 420 N, 75° 330 E) with an expected average solar irradiance of 5.5 kW m-2. Average cost of drying one batch of surgical cotton in a solar tunnel dryer has been worked out to be approximately 4.63 USD as compared to 30.00 USD in the existing diesel fired dryer. Keywords Solar tunnel dryer Surgical cotton Natural convection Drying time Nomenclature W Mass of product, kg Cp Specific heat of water, kJ kg-1°C-1 Ca Specific heat of air, kJ kg-1C-1 Td Drying temperature, °C
Ta Te Mw mw k mf mi td Q Qt Qa Qc r L Rham td I g qa qe Da Di
Ambient temperature, °C Temperature of moist air at chimney outlet, °C Mass of water to be removed during drying, kg Mass of water to be removed per hour, kg h-1 Latent heat of vaporization, kJ kg-1 Final moisture content, % Initial moisture content, % Assumed drying time, h Total energy required, kJ Energy required per hour, kJ h-1 Rate of air required, m3s-1 Rate of air flow in chimney, m3s-1 Radius of dryer (m) Length of dryer (m) Ambient relative humidity, % Drying period, h Incident solar radiation, MJ h-1 m-2 Dryer efficiency Density of air at ambient temperature, kg m-3 Density of exit air, kg m-3 Actual draft Produced draft
Introduction N. S. Rathore College of Dairy and Food Science Technology, Maharana Pratap University of Agriculture and Technology, Udaipur, Rajasthan 313 001, India N. L. Panwar (&) Department of Renewable Energy Sources, College of Technology and Engineering, Maharana Pratap University of Agriculture and Technology, Udaipur, Rajasthan 313 001, India e-mail:
[email protected]
Petroleum energy now supplies most of the energy for production in agriculture and agro-based industries. As the conventional petroleum fuels become scarce and expensive (140 USD per barrel, 30 June 2008), the use of these high energy content fuels in low temperature stationary uses, such as environmental temperature and humidity control, crop drying, water heating, and irrigation must be restricted. Energy for all these uses can be supplied by solar energy (Arinze 1986; Rathore et al. 2007).
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Open sun drying is one of the oldest techniques employed for processing agricultural and food products. Open sun drying has been traditionally practiced in India for drying agricultural products. Considerable savings can be made with this type of drying since the source of energy is free and sustainable. However, this method of drying is extremely weather dependent and has the problems of contamination, infestation, microbial attacks etc., thus affecting the product quality. Additionally, the drying time required for a given commodity can be quite long and results in post-harvest losses. Solar drying of agricultural products in enclosed structures by natural convection is an attractive way of reducing post harvest losses and low quality of dried products associated with traditional sundrying methods (Muhlbauer 1986; Forson et al. 1996; Bena and Fuller 2002; Chua and Chou 2003; Sacilik et al. 2006; Forson et al. 2007). Several attempts for developing natural convection solar crop-dryers (both of the cabinet-type and the tunnel-type) have been investigated and experimented, over the years, and are described in the literature (Bassey et al. 1986; Bena and Fuller 2002: Hossain et al. 2005; Hossain and Bala 2007). The crop is vulnerable to damage due to hostile weather conditions. The crop is also susceptible to reabsorption of moisture, if it is kept on the ground during periods of no sun, which reduces its quality. This type of problem can be solved by solar dryer (Maddhlopa et al. 2002). Many dryers have been developed and used to dry agricultural product in order to improve shelf life (Esper and Muhlbauer 1996; Chen et al. 2005). Most of these either use expensive sources of energy such as electricity (El-Shaitry et al. 1991) or combination of solar energy and some other form of energy which is not cost effective and dependent on regular supply of fuel (Rathore and Panwar 2007). To overcome this problem an energy efficient solar tunnel dryer has been designed and developed by the Maharana Pratap University of Agriculture and Technology (MPUAT), Udaipur, Rajasthan, India Subsequently, testing of this dryer for agriculture, horticulture and chemical products on commercial scale was also conducted to find out operational difficulties and overall energy conservation at individual level.
Principles of drying Drying is essentially the removal of moisture from produce for safe storage by the application of heat. The moisture content in chemically treated surgical cotton is usually 35–40% on wet basis. In the natural convection type solar tunnel dryer, air is heated inside the dryer due to greenhouse effect by natural means. Increased drying process takes place by hot air flow, and the passage of an air mass around a product represents a complex thermal process
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Fig. 1 Drying process illustrated on psychometric chart
where unsteady heat and moisture transfers occur simultaneously. Heat and moisture transfer removal rate depends on air velocity and temperature of the circulating drying air (Sahin and Dincer 2002). In natural convection type dryer, air velocity depends on created draft because of temperature difference. Figure 1 reveals that as hot air circulated over produce, partial pressure difference is created inside the produce. This pressure difference is responsible for movement of moisture from inner core of the produce to its surface. As moisture starts evaporating, relative humidity is increased and evaporated moisture is carried away by air (Van Arsdel and Copley 1963)
System description of natural convection solar tunnel dryer Figure 2 shows a schematic view of a natural convection solar tunnel drying system, designed and developed for drying Agro-Industrial products (surgical cotton) on large scale at moderate temperature in this region. A battery of three solar tunnel dryers has been commissioned at M/s Cotton Products of India, Village Lakkadvas, Udaipur (27° 420 N, 75° 330 E) for drying 1,800 kg surgical cotton from 40% moisture content to 5% moisture content in one batch (see Plate 1). Earlier electricity was used for air heating and blowing inside the drying chamber. The system consists of three solar tunnel dryer of size 18 m 9 3.75 m placed adjacent to each other accommodating 600 kg of surgical cotton in one dryer. It is consisted of a hemicylindrical walk-in type metallic frame structure covered with UV stabilized semi-transparent polyethylene sheet of 200 l thickness. A slope of 2–3° along the length of the
Energy efficient solar tunnel dryer for industrial drying
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Fig. 2 Schematic view of natural convection solar tunnel dryer
air entry into tunnel. This fresh air gets heated and owing to density difference moves upward through the products and picked up moisture and finally passed out through chimneys. Salient features of solar tunnel dryer
Plate 1 Battery of solar tunnel dryer
tunnel and an exhaust fan of 0.75 kW power rating and having volume flow rate 1,500 m3h-1 is provided at the upper end of the tunnel for occasional removal of moist air to maintain humidity at a preset level inside the tunnel. The exhaust fan is coupled with automatic Relative humidity control meter with ‘NOT’ electronic gate, which operates only above 35% and closes at 15% relative humidity value. Adequate provisions were made to reduce heat losses from the floor and the northern side of the tunnel. The product to be dried is put into a continuous perforated tray in thin layers. Four chimneys were placed on equidistance apart on the top of tunnel to remove moisture during drying. On the bottom side of southern side, four holes having equal diameter as that of chimney were provided to allow fresh
Solar tunnel dryer is a simple, walk-in type, convenient and efficient dryer at low cost for drying large quantity. It is essentially a poly house type structure having loading capacity up to 600 kg of surgical cotton per batch, in which drying takes place through natural flow of hot air. It is hemi-cylindrical tunnel structure in shape where loading and unloading of material is quite easy (see Plate 2). Few salient features of the solar tunnel dryer are listed below. 1. 2. 3. 4. 5. 6. 7.
Simple in construction and control of temperature and humidity is easy. Loading and unloading of material on large scale is quite easy. Cheaper model for drying of commodity on large scale. Moderate design according to specific needs. Local artisan can fabricate the system with locally available cheaper materials. All industrial and agriculture products having free moisture can easily be dried. This is new concept for integrating solar energy on large scale.
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The mass of water to be removed during drying, Mw kg mi mf W Mw ¼ 100 mf Mass of water removed per hour mw, kg h-1 mw ¼
Mw td
Total energy required Q kJ Q ¼ W Cp ðTd Ta Þ þ ðMw kÞ Energy required per hour Qt kJ h-1 Qt ¼
Q td
Drying area Plate 2 Inside view of solar tunnel dryer
8. 9.
There is a good scope for entrepreneurship development in its implementation. All industrial and agricultural products can be dried with very low cost of operation (exhaust fan operating cost only).
Design of solar tunnel dryer
It has been observed that about 68% of the area of hemispherical-shaped solar tunnel dryer toward south is able to receive sunlight where as remaining 32% of the area toward north is shaded from the sun. Global solar radiation (I) for Udaipur region is 5.5 kWh m-2. The floor area of dryer is calculated as follows Qt Area ¼ ðI g 0:68Þ
To carry out design calculation and size of the tunnel dryer, the following assumptions and conditions were made as summarized in Table 1.
Size of STD required Energy gainer area is about 68% Area of hemispherical-shaped solar tunnel dryer = p rL Diameter 3.75 m is kept constant for easy entry and other convenience
Table 1 Design conditions and assumptions
Design of North wall
Items
Condition or assumptions
Location
Udaipur (27° 420 N, 75° 330 E)
Final moisture content (mf)
5%
Area shaded from sun is about 32% Total area of north wall to be protected Ap = drying area 9 32% Perimeter of north wall (P) = p r Since perimeter (P) cover diametrical length (Lp) = 3.75 m Arc width of cover through which energy losses (w) = Ap/L Arch width (w) will cover diametrical length (Lp1)
Ambient air temperature (Tam)
32°C (Average for April, May–June)
Lp1 ¼
Ambient relative humidity (Rham)
25% (Average for April, May–June)
Maximum allowable temperature
65°C
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Required length of protector hp ¼ ðwÞ2 ðLp1 Þ2
Sunshine hour
10 h
Drying period (Td)
8h
Incident solar radiation (I)
5.5 kWh m-2
Dryer efficiency (g)
30%
Product
Surgical Cotton
Loading rate (M) in each batch
600 kg
Initial moisture content (mi)
40%
Thickness of plastic sheet
200 l (UV stabilized)
Density of air at ambient (kg m-3) Height of chimney (H)
1.252 2.40 m
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Lp w P
Design of chimney Quantity of air needed to absorb mw kg of water mw k Qa ¼ Ca qa ðTe Ta Þ Now, Qa amount of moist air is needed to be removed in 8 h.
Energy efficient solar tunnel dryer for industrial drying
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Table 2 Dimensions of solar tunnel dryer Quantity of water to be removed
253 kg
Drying area
92.48 m2
Diameter of dryer
3.75 m
Length of dryer
18.00 m
Width of north wall
1.88 m
Height of north wall
1.45 m
Number of chimney
4
Height of chimney
0.40 m
Diameter of chimney
0.20 m
¼ Qa =ð8 60 60Þ; m3 s1 Draft produce if we assume height of chimney by 2.40 m (four numbers of 0.60 m)
winter months of 2008. The system is oriented to face south to maximize the solar radiation incident on the dryer. The global solar radiation incident on a horizontal surface is measured using an Eppley–Precision Spectral Pyranometer. Wind speed and exit air velocity at chimney outlet are measured by using Lutron Anemometer Model no. AM-4822 and hot wire anemometer Model no. LM-4204, respectively. Calibrated NiCr–Ni thermocouples connected to a multi-channel Emcon—Digital Solar Data Monitor (Environmental Measurement and Control, Cochin) are used to measure the temperatures at different locations of dryer, e.g. centre, southern, northern, floor and ambient. The temperatures of the different parts of the system are measured after every one hour. The ambient temperatures were also recorded; the moisture contents of product during drying are measured on the dry basis.
Di ¼ H gðqa qe Þ 9 ffi Di But actual draft Da = 0.40 qffiffiffiffiffi a Velocity of exit air, v ¼ 2D qe Thus if assumed this exit air is being carried out by four chimneys Qa 4
Area of chimney Qc V K
Dimension of Solar Tunnel dryer The solar tunnel dryer was designed as per the procedure mentioned above, and its dimensions are given in Table 2.
Instrumentation and experiments For drying surgical cotton at industrial level experiments were conducted during the typical day of summer and
Fig. 5 Temperature variation at no load (15 April 2008)
The testings on no load and full load were made for winter and summer seasons. The average temperature inside the tunnel was 18–20°C higher than the ambient temperature. The moisture content of the surgical cotton was reduced to around 5% from an initial value of 40% in one single solar day. Temperature variation during no load testing in natural convection solar tunnel dryer on a typical day (Duffie and Beckman 1991) of summer month (15 April 2008) and winter month (17 January 2008) at different locations inside the tunnel dryer and the outside is graphically illustrated in Figs. 5, 6, respectively. It is observed from the Fig. 5 that the maximum temperature inside the solar tunnel dryer on a typical day was 54.6°C at 1.30 PM while the minimum temperature was 28°C at 9.00 AM on summer day. The maximum ambient
Centre
Southern
Northern
Floor
Ambient
60 50 45 40 35 30 25 20
.0 0 18
00 17 .
16 .0 0
15 .0 0
13 .5 0
.0 0 13
00 12 .
0
9. 00
00
15 8.
o
Temperature C
55
11 .0
A¼
Performance of the dryer
10 .0 0
Qc ¼
Result and discussions
Time (h)
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N. S. Rathore, N. L. Panwar
Fig. 6 Temperature variation at no full load (17 Jan 2008)
Centre
Southern
Northern
Floor
Ambient
45
35
o
Temperature C
40
30 25 20 15
0 .0
0 17
16
18
.0
0 .0
0 .0
0 13
13
15
.5
0 .0
0 .0 12
11
10
.0
.0
0
0
00 9.
8.
00
10
Time (h)
Centre
Southern
Floor
Ambient
.0 0 18 .0 0
17
0 .0
16
0
0
.0 15
.5 13
0 .0
13
.0
0
0 .0
12
11
0 .0 10
00
00
9.
8.
Time (h)
Fig. 7 Temperature variation at full load (15 May 2008)
Moisture content
60
44
50
34
40 24 30 14
20 10 7.00
9.00
11.00
13.00
15.00
17.00
4 19.00
Moisture content, %
Inside dryer
Time, h
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Northern
55 50 45 40 35 30 25 20 15
Ambient
Relative humidity,%
Fig. 8 Variation with relative humidity and moisture content (15 May 2008)
relative humidity to maximum value of about 55% at 11:00 h and moisture content reduction from 40 to 26%. As moisture starts evaporating from product, it reduced the moisture content hence decreasing trend of the relative humidity was recorded after 12:00 h. Relative humidity inside the dryer varied in the range of about 21–55% whereas ambient relative humidity was in the range of about 14–18%. The required moisture content from 40 to 5% obtained in 8 h of operation.
Temperature oC
temperature recorded on the summer day was 35.8°C at 1.30 PM, while minimum ambient temperature was 23°C at 9.00 AM. It is observed from Fig. 6 that the maximum temperature inside the solar tunnel dryer during no load was 42.1°C at 14.00 h, while the minimum temperature was 15°C at 8.00 h in winter. Maximum ambient temperature on a typical day of winter season was 24.8°C at 14.00 h while the minimum was 11.1°C at 8.00 h. In full load testing cotton was placed in solar tunnel dryer and some materials were placed in open air for comparison purpose. Drying was continued till the moisture content of the material reached to 5% in both conditions. The maximum temperature inside the solar tunnel dryer in full load test was 53.8°C at 14.00 h while the minimum temperature was 25°C at 8.00 h during summer day (15 May 2008) as illustrated in Fig. 7. The maximum ambient temperature recorded on this day was 35°C at 14.00 h and minimum ambient temperature was 22°C at 8.00 h. Figure 8 reveals that during full load conditions initially wet products get heated inside the dryer thus increasing the
Energy efficient solar tunnel dryer for industrial drying Table 3 Detail of the costs of solar tunnel dryer
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S. No.
Item
Quantity
1.
Galvanized Iron pipe 15 mm class A
120 m
285.00
2.
Galvanized Iron pipe 25 mm class A
50 m
117.00
3.
Exhaust Fan with automatic humidity controller
One
185.00
4.
Metallic Door on both sides with stands
Two
116.00
5.
200 l UV stabilized polythene sheet
50 kg (250 m2) 2
2
6.
Pucca floor with black paint (20 9 5 m )
100 m
7.
Insulation inside the floor
5 cm thick
8.
Drying beds of 2.75 m wide having
Cost (USD)
228.00 320.00 115.00 231.00
7.5 cm depth 9.
MS Sheet Sandwich with insulating material for north wall and metallic chimney with stand
76 m2 plus six chimney
230.00
10. 11.
Skilled Labour for fabrications Miscellaneous (cement, nut bolt, cable, wire etc.)
30 man days
127.00 46.00
Total cost of Installation
Cost of operation Cost for material and construction of the solar tunnel dryer for drying 600 kg of surgical cotton in a dryer was estimated as around USD 2000.00 (1 USD = 43.15 INR, 28 June 2008). Average cost of drying for one batch of surgical cotton in one solar tunnel dryer was worked out approximately as 4.63 USD as compared to 30.00 USD in the existing diesel fired dryer. The cost of construction of solar tunnel drying system and its economics is presented in Tables 3 and 4, respectively.
2000.00
(other places as well) is working efficiently for drying surgical cotton of 600 kg in a batch. The system is saving about 25.37 USD per day. There is need to integrate solar tunnel dryer for drying agriculture, horticulture and industrial products on large scale at different agro-industrial levels. Further, this technology can contribute in clean development mechanism and thus organization effectiveness and higher productivity (Jain 2007) and along with long-term benefits in terms of fuel and environment saving can be obtained.
References
Conclusions Renewable energy sources are mostly indigenous and traditional sources, which are regenerated during the annual solar cycle. Technologies for their conversion and utilization are environmentally friendly. Experience in similar solar dryer for fruits and vegetables proved that solar drying can be attractive method for food preservation and also for a commercial proposition. Drying conditions in Udaipur, Rajasthan, India are very favourable, and initiation of a solar dryer can save conventional fuel. Natural convection type solar tunnel dryer developed at Udaipur
Table 4 Economics of Surgical Cotton dried in solar tunnel dryer Initial investment
2000.00 USD
Total cost of drying through diesel fired dryer for drying 600 kg in a batch
30.00 USD
Cost of drying through solar tunnel dryer including cost of labour etc.
4.63 USD per day
Savings
25.37 USD per day
Payback period of dryer
79 working days
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