INVESTIGATION OF THE SOLAR ENERGY UTILIZATION FOR MEETING PART OF THE THERMAL DEMANDS OF AGRICULTURAL-PRODUCT MECHANICAL DRYERS
Stamatios Babalis, Elias Papanikolaou, Vasilios Belessiotis
Solar & Energy Systems Lab, NCSR “Demokritos”
153 10 Ag. Paraskevi Attikis - Greece
sollab@
ABSTRACT
In this study a methodology for the determination of the fraction of thermal loads supplied by a solar energy system, employed for preheating air in a mechanical dryer, is prented. The basic parameters influencing the mechanical-dryer energy supply system under steady-state operating conditions are studied, and the analytical relationships between the energy quantities and the drying conditions are also determined. Therefore the thermal loads for drying raisins and figs at three different air temperatures are calculated. The energy to preheat the drying air is provided by a solar collector field, connected to a heat exchanger directly inrted into the airflow. Using the f-chart method, suitably appli
supermarket的读音ed and taking into account the appropriate designing parameters, the fraction of the dryer thermal loads covered by the solar energy system is determined on a daily basis. This investigation gives ri to the conclusion that for drying temperatures of 70÷75 o C (typical for grapes and figs) a significant fraction of the thermal loads of the mechanical dryer could be supplied by a solar system depending on the solar collecting area.
1. INTRODUCTION
webtrendsThe fundamental benefits of the traditional, open-air solar drying consist in the utilization of a free and clean resource, solar energy. The direct solar energy is mainly available during the summer period, coinciding practically with the harvesting period of a large number of agricultural products. The intermittent nature of the solar energy supply and the physical contact of the dried product with the ambient air are prevented by using heated airflow in mechanical drying. Moreover energy demands for the successful completion of the drying process could be accomplished either by using conventional sources (liquid and gas fuels), or renewable ones such as solar energy. Due to the intrinsic intermittent nature of solar energy, only part of the necessary thermal demand may be possibly covered, justifying in that way the u of a supplementary source (hybrid systems). The percentage of energy demands covered by each source should be investigated by considering cost
efficiency criteria. [2] [5]
The fraction of thermal demands covered, during the daytime, by means of solar energy systems is strongly dependent on veral parameters. Having in mind that a solar system is not necessarily dedicated to one and single purpo, hot water supply for domestic or industrial purpos come along to improve the system efficiency. The coverage fraction of the dryer thermal demands is the determinant factor in the efficient sizing of a solar system, as well as component lection. [1]
In the prent study, the thermal demands of a typical, 300 kg capacity, mechanical dryer, are initially estimated. As a basis for the calculations, the agricultural-product mechanical dryer developed and tested by the NCSR “Demokritos” has been considered. The thermal demands are determined at three drying temperatures, 50o C, 60o C and 70o C for raisins and 55o C, 65o C and 75o C for figs. For
Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement 1982
the drying conditions, experimental results for 2 m/s air velocity are available and the expected drying time could be determined. Part of the thermal needs required for drying are covered by water heated from solar collectors. The heat fraction covered by the solar system has to be determined usi
机械设计制造及其自动化英文ng the f-chart method, considering the Athens area meteorological data.
Having a good idea of the percentage of the thermal demands expected to be met daily, uful conclusions could be deduced from this technical analysis concerning the viability of the introduction of a solar system for the heating of drying air.
2. DETERMINATION OF THE THERMAL DEMANDS OF A MECHANICAL DRYER
2.1 Energy Aspects of the Drying Process
Drying of a food product, considered as a moisture reduction technique by means of thermal methods, is a complicated process involving mutually related transfer phenomena.
As well known, drying rate of a certain product is not to be considered as constant during the entire drying process. Initially a short time period of increasing drying rate occurs (pha Ι), known as warming up period. Subquently, a constant drying-rate period occurs (pha ΙΙ), and the moisture moving from the pores of the product to the surface evaporates, while the product temperature remain constant. Free water is evaporated during this time period. Finally (pha ΙΙΙ), the drying rate decreas becau the quantity of moisture actually removed becomes less than what could theoretically be evaporated with the heat supplied.
As a matter of fact, pha ΙΙΙ, namely the falling rate period, energetically reprents the most interesting period becau water bonded with the solid matrix is forced to evaporate. As the moisture content diminishes and therefore water-bonding energy rais, higher energy amounts have to be supplied to achieve the evaporation. Energy consumption during the falling rate period is substantially higher than that of the other two and of decisive importance for the control and energy savings. The temperature influence becomes more important during the final drying period (bond water evaporation) unlike the influence of heated-air velocity, the importance of which becomes greater in the first stage of drying (evaporation of the large amount of free water).
From the aforementioned analysis, the strong influence of the drying conditions, mostly air temperature, on the thermal demands becomes evident and the determination of the optimal drying conditions turns out to be of paramount importance for each specific product.
For the needs of the prent work the following assumptions have been adopted: () The energy of desorption of the bonded water is constant during drying and equals the latent heat of evaporation of free water, and (2) the product temperature during drying is constant and equals the air temperature. The fresh materials lected are typical agricultural products of Greece (Figs and grapes) the drying curves of which are available in literature. [4] The drying time of grapes at each respective temperatu
re has the following values:
Τair = 50 o C 60
o C 70
o C
Drying Time = 54 h 33 h 12 h
The correspondent drying time for figs has the following values:
Τair = 55 o C 65
西安培训学校o C 75
o C
Drying Time = 26 h 16 h 12 h
2.2 The Typical Mechanical Dryer
The investigation of the dryer thermal demands was bad on the tunnel tray type mechanical dryer with a capacity of 300 kg of fresh product, developed and tested at the Solar Laboratory of NCSR “Demokritos”. The fresh product is placed on the trays and dried by a stream of preheated air coming parallel to the surface. The typical mechanical dryer is depicted in fig. 1 the main parts of which could be summarized as follows:
Drying chamber, consisting of a thermally insulated compartment having a system of supporting the fresh product trays usually in stacks, the number and size of
5 SOLAR THERMAL SYSTEMS AND APPLICATIONS 1983
which determines the dimensions of the chamber and
the capacity of the dryer.
A system of handling and distribution of the drying air, consisting of one or more fans and system of ducting and air plenums.
The drying-air heating and preheating system, consisting of the main and auxiliary heating system as well as of the preheating and recirculation energy-saving systems. A system of dumpers and duct
s allowed the recirculation of part of the drying air. The fresh air was preheated by the hot air stream drawn off. Automatic control and measurement system for the measurement and control of the main thermodynamic quantities.
FIELD
HEAT SOURCE
DRYER
Fig. 1: Schematic diagram of a typical mechanical dryer for
agricultural dryers. All the parts, along with the drying chamber, ud to handle the hot air are therma
lly insulated, contributing in this way to the energy savings of the overall system. The energy to preheat the drying air is provided by a solar collector field, connected to a heat exchanger directly inrted into the airflow. The additional thermal energy has been supplied by a conventional heat source.
2.3 Estimation of the Thermal Demands
The thermal demands for the operation of a dryer could be calculated by adding the partial thermal needs consumed for heating up the various components of the dried material and the heat loss of the dryer.
An amount of heat pr Q is needed to increa the temperature of the product of mass pr M from the initial ambient temperature amb T to the final drying state dr T :
()pr pr pr dr amb p Q M C T T =⋅⋅− (1)
The specific heat of the product pr p C with respect to the moisture content pr mc was calculated by: [3]
pr pr 1.4 2.97p C mc =+⋅ (2)
腐蚀性
whereas an amount of heat .ev wat Q needed for the evaporation of the quantity of water removed .ev wat M by:
..ev wat ev wat wat Q M L =⋅ (3)
where wat L is the latent heat of evaporation of water. The mass of water evaporated during drying is:
()
ev.wat pr fr.prod dr.prod ΜM mc mc =⋅− (4)
with fr.prod mc and dr.prod mc the moisture content of the fresh and the dry product respectively. By substitution of (4) in (3) we have:
()
...ev wat pr wat fr prod dr prod Q M mc mc L =⋅−⋅ (5)
The drying rate is considered constant during drying.
The amount of heat fr.air Q for increasing the temperature of a mass air M of fresh air entrained during the recirculation (considered R = 90%) from ambient to dr T can be estimated by:
奖牌英文
()fr.air air air dr amb (1p Q R)M C T T =η⋅−⋅⋅⋅− (6)
The efficiency of the air-air heat is considered to be η= 65%.
Considering the dryer in steady-state operation the heat loss could be attributed to tho through the walls by convection and to air leakages. The heat loss through the walls wall Q , depend on the materials ud for the specific application, which in our ca consisted of an internal steel sheet (s 1=0.8 mm, λ=55 W/m K) an insulating glass wool plate ( s ins =50 mm και λins = 0.03 W/m K) and an external aluminum sheet (s 1=0.5 mm, λ=200 W/m K). Thus, the heat loss could be summarized in:
()tot wall dr amb Q U A T T =⋅⋅− (7)
having a total loss coefficient
tot U :
Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement玩笑的英文
1984
insul 12tot 0121
insul s s s 111
U h h λλλ=++++ (8) with dryers side area A and internal and external convection coefficients 0h and 1h :
Finally the heat loss due to the inefficient air-tightness of the whole structure could roughly be estimated at 10%. Initially the thermal demands are expresd in degree hours and the thermal load due to water evaporation was calculated for a typical hour of operation, whereas finally the thermal demands were expresd per day of operation.aptech
3. THE SOLAR SYSTEM
For the determination of the fraction of thermal load to be met by the solar system, the f -chart method has been applied. [1] The system for heating drying air, depicted in Fig. 2, consists of the following parts:
Solar Collector Field, consisting of a typical flat-plate solar collector with lective absorber surface, having the following characteristics:
F R U L = 4.5 W/o C m 2
and F R (τα)n = 0.8 Heat Exchanger betw een Storage and Solar Field , having a correction factor: F’R / F R = 0.95
Main Water Storage Tank, for the storage of solar energy, with a capacity of 75 lt for any sq.. m. of collecting surface. The hot water from the main storage is forced to circulate in an air-water heat exchanger heating up the air of drying, the main portion of which is recirculated inside the dryer.
Fig. 2: Schematic diagram of a typical solar heating system.
The additional energy required has to be supplied by a conventional source.
The thermal loads, in degree-hours, replenishing the loss of the dryer are estimated in the premis of Athens. The investigation refers to the operation at three different drying temperatures of the dryer, for typical agricultural products (figs and grapes) and during a typical day of August, who conditions are also applicable to September or July.
4. RESULTS AND CONCLUSIONS
In fig. 3 and 4 the results from the investigation of the dryer operation are prented. They refer to three different drying temperatures and two different products, considering the mean covering ratio of the heat demands with respect to the collecting area of the solar field.
By examining the diagrams it is evident that a considerable amount of the thermal demands per day could be met by solar energy, even with relatively small collecting surface area, for both grapes and figs. Having a collecting area of 100 m 2 a 70% of the thermal needs for drying figs and ~65% for drying grapes could be covered by the solar field (in 70÷75 o C). Even a small solar collector field ud for the different needs of a rural ur could be considered for drying the agricultural products harvested during harvesting time. In lower temperatures a small part of the thermal demands could be covered, considering therefore an extension of the drying time rather than a temperature
increa.
Fig 3: Fraction of the thermal load supplied by the solar
system for drying Figs.
5 SOLAR THERMAL SYSTEMS AND APPLICATIONS1985
Fig. 4: Fraction of the thermal load supplied by the solar system for drying grapes.
Bad on this investigation, it may be concluded that the cooperation of relatively small solar collector fields with a hot-air mechanical dryer could be beneficial to conrving energy while maintaining product quality. 5. REFERENCES
(1) Duffie J.A. and Beckman W.A., (1991). Solarchine guys video
engineering of thermal process. John Wiley.
(2) Imre L.L., (1987). Solar drying. In Handbook of
industrial drying, edited by Mujumdar, A.S., McGill University, Montreal Canada.
(3) Moshenin N. (1987). Thermal properties of foods and
Agricultural materials. Gordon and Breach, London. (4) Karathanos V.T. and Belessiotis V.G., (1997). Sun and
artificial air drying kinetics of some agricultural products. J. Food Engin., 31(1): 35-46.
(5) Wisniewski G.,D., (1993). Methods of Rational Solarforklift
Collectors Selection for Crop Drying. Proceedings of Is Solar World Congress Budapest, V olume 8.
