CC BY-NC-ND
0Available online at www.sciencedirect.com
V ScienceDirect
Energy Procedia 6 (2011) 583-591
MEDGREEN 2011-LB
Energy and exergy analysis of solar drying process of Mint Amel BOULEMTAFES-BOUKADOUM1 , Ahmed BENZAOUI2 a*
laboratoire Solaire Thermique et Géothermie, Centre de Développement des Energies Renouvelables, B.P. 62 Route de
l'observatoire, Bouzaréah, Alger - ALGERIE
2Laboratoire de Thermodynamique et des Systèmes Energétiques, USTHB, Bp. 32 El Alia, 16111 Alger Algérie
Abstract
Renewable energy in food industry and particularly in drying process is growing and mainly in developing countries. Solar energy is often used by direct products exposure through a glass or to heat drying air through a solar collector. However, the random and intermittent nature of solar radiation leads to use conventional energy sources as supplement.
Hence, optimization and design tend to reduce the drying time of products for minimum possible energy use. These cases are often preceded by rigorous energy and exergy drying process analysis. That is the aim of our work in this paper.
The used dryer is an indirect type, passive, without extra energy and discontinuously operating. It is composed of a solar air heater and a drying room. The experiment took place at Bouzareah on the heights of Algiers in the summer season. The choice of mint is because its abundance and its wide use in Algeria. Using the first and second principles of thermodynamics, we could estimate useful energy received by the heater and that really used during drying. Energy analysis has allowed us to quantify the solar energy received by solar heater and available for drying. Exergy analysis has allowed us to estimate the energy losses during the drying process.
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]
Keywords: Energy analysis, exergy, solar energy, drying, solar dryer , efficiency
Nomenclature
A solar air heater area. m2 Cp heat capacity. kj/kg.K
* Corresponding . Tel.: +213 21 90 15 03; fax: +213 21 90 15 60. E-mail address: a_boulemtafes@cder.dz, aboukadoum@gmail.com
1876-6102 © 2011 Published by Elsevier Ltd. doi:10.1016/j.egypro.2011.05.067
Ex exergy.kj/kg
g gravitation. m/s2
G tilted global solar irradiation.W/m2'
h Enthalpy .Kj/Kg
m mass flow rate kg/s
P atmospherice Pressure. kPa
Q net heat rate kj/s
S Entropy. Kj/Kg. K
T température, K
U internal Energy. Kj
V velocity. m/s
V volume.m3
w spécific humidity. g/g
w energy utililization rate. kj/s
greec .symbols
0 relative humidity%
S exergetic efficiency %
Subscripts
e inlet
as air drying
s outlet
ac solar heater
amb ambiant
0 ambiance
l loss
sat saturation
1. Introduction
The solar heater is a particular type of heat exchanger allowing the conversion of received solar radiations to thermal energy. In opposition to the conventional exchangers where the radiation transfer is
negligible, it is the main transfer mode in the solar heater where the solar energy converted into thermal one is transferred to flowing air. However this rate of transfer remains rather weak in comparison to solar concentrators and that remains the major disadvantage of the flat plate solar heaters [1].
Among the most widespread applications of solar heaters, one can note water domestic heating, space buildings heating and the solar drying of agricultural products. However, randomness and intermittent characteristic of solar radiation motivates the use of conventional energies sources as supplement. That's why precise optimization dimensioning studies, are required in order to reduce the drying time and energy use. Those studies must be preceded by rigorous energy and exergy analysis of the drying process, which is the aim of this work.
2. Experimental Setup
The experiments have been done at C.D.E.R site (Centre of Renewable Energy Development) (Latitude 36°8 North, Longitude 3°12 east , Altitude 345m, Albedo 0.2) on summer season. The solar air heater tested and used [2] for mint dying is made up:
■ A flat plate solar collector that heats the drying air at desired temperature. It is a simple circulation and simple glazing collector with external dimensions (2mx1mx0.1m). The air is flowing between absorber and bottom of the collector inside wood corridors.
■ A parallelepiped box drying, made in galvanized metal sheet, insulated from the indoor. The products to be dried are laid out on trays allowing the passage of the air. Device operates in natural convection and in a discontinuous way (diurnal).
Measurements have been carried out during experiments [3]:
• A pyranometer (Kypp Zonnen) is used to measure tilted global solar radiation every hour .
• Temperature and relative humidity of the drying air are measured in various locations of the device.
• The drying air flow rate is determined after measuring air drying velocity .
• The mass loss curve of mint is drawn afterwards successive weightings of the product.
3 Energy Analysis
Drying is a very complex process whose goal is to remove partially or completely the moisture contained in a product. This process involves a double transfer of heat and mass, thus it is a very inefficient operation. The energy needed for drying depends mainly on the nature of the product to be dried and the drying rate. Hence the usefulness of an energy and exergy analysis for each product in order to quantify the energy needed for drying and to locate the exergy losses in each step of the process. Theoretical and experimental studies of drying process are necessary; those concerning energy and exergy aspects of the process are more interesting.
In this work, we give an overview of the results of energy and exergy analysis of mint drying process using solar energy. Energy analysis of the drying process of mint is made on the basis of mass and energy equations, in steady state [4]:
Fig. 1 The indirect solar dryer
Mass conservation equation the drying air
Zm = V m (1)
ase ass
Moisture conservation equation of the drying air
V (m wee + mxp ) = V m es
V (m + mxp ) = V m JW, (2) Energy conservation equation
Q-W = V m ^ {hass + V 2 -A 1 - V m ^ {hase + V V) (3)
3.1 Thermodynamic parameters
Wet air is considered as one phase homogeneous system with only two components governed by ideal gas law for fluid mixtures.
3.1.1Relative humidity
It is defined as the ratio between the partial vapour pressure of water in the mixture at a given temperature (pv,T), and the saturated vapour pressure at the same temperature (psat,T):
xl00 (% ) (4)
(pa, , T )
3.1.2 Specific humidity
Defined as the water vapour mass per drying air unit mass
W = —^ = 0.622
m „.
P - Pvj
3.1.3Enthalpy (of the drying air) It is expressed by [5]:
has = C pasTas + wh sat ,t (6)
where Cpas defines the specific heat of drying air, Tas is the drying air temperature, w is the specific humidity and hsatT is the enthalpy of the saturated vapour.
3 2 Determination of the outlet conditions of the solar air heater
It is considered that the conditions of entry are those of the ambient conditions.
3.2.1 Useful energy received by the collector [1]
Q cas = mas C pas (Tacs - Tace ) (7)
3.2.2 The instantaneous efficiency (performance) of the solar air heater It is given by the following relation:
Qca^ (9)
3 3 Determination of inlet and outlet conditions of the drying room:
It is considered that the characteristics of the air at the entry of the box of drying (temperature, moisture) are those of of its solar collector exit [5]. T = T
A ase A acs
Wase = ^s (10)
Using the equations (1) and (2), one obtains
W = W + ^ (U)
ass ase .
The relative humidity and the enthalpy of the air of drying at exit of the drying room are given determinated from the equations (4) and (6).
3.3.1 Energy used
The energy required to transfer moisture during the drying process (inside the drying box) can be quantified by the following relation
Q as = mas (hase - hass ) (12)
4. Exergy analysis
Based on the second law of thermodynamics, we calculated the exergy at the inlet and outlet of the dryer and the exergy loss. Exergy analysis is based on the determination of exergy values at different points in the process steady. Those are calculated on the basis of thermodynamic parameters from the first law of thermodynamics. Exergy is defined as the energy available and actually converted into work [4]
Ex=Eph+Ekn+Ept+Ech (15)
Eph Physical exergy, Ekn Kinetic exergy, Ept Potential exergy, Ech Chemical exergy expanding terms that gives:
Ex= (U—U0)+P0(V-V0)-T0(S-S0)+1/2mv2 +mgz+(ER+EN) (16)
In considering the conditions, we neglect some terms and found [5]:
Ex=maCp[(T-T0)-T0 ln(T/T0)] (17)
®ase = ®acs
hase = hacs
From the equation (15), one can calculate the inlet and outlet exergy as well as the loss of exergy. These are given by [6]:
EXe=maCp[(Tase-To)-To ln(Tase/To)] (18)
EXs=maCp[(Tase-To)-To ln(Tase/To)] (19)
Exl=Exe-Exs (2o) One defines also the exergy effectiveness as being:
e =1-(Exl/Exe)=Exs/Exe (21)
5. Results and discussion
The energy and exergy analyses of the thin-layer drying process of mint via an indirect solar dryer were performed with data obtained from the experiments .The curves representing the drying experimental results as well as energy and exergy analysis are given below. In the figure1 (a,b.), we show global irradiation evolution and useful energy received by the solar air heater curves. The experiment was conducted during sunny days with light clouds. The values of global irradiation vary between 4oo and 85o W/m2 with a peak around 13H local time. One can note similarity in curve shape of useful energy received by the solar heater and global irradiation. It was observed that values of the useful energy during the first day were although similar or even higher than that of the second. Finally, let us note that the maximum recorded was 45o J/s in the first day and 3oo J/s in the second.
(a) (b)
Fig. 1 Tilted global solar radiation received by the solar heater and useful energy (a 1st day, b 2nd day)
In the figure 2 (a.b), the evolution of instantaneous solar air heater efficiency (performance) is given for the two days. The values vary between 1o and 3o %, which is very acceptable, with higher maximumvalues the first day. These values reflect the rate of solar energy received by the collector and actually transferred to the coolant (air).
Fig. 2 Instantaneous performance of the solar heater (a 1st day, b 2n day )
Figures 3 (a.b) show the evolution of the energy used to dry mint samples over the two days of drying time. We note this energy is more important the second day, in our opinion this due to the internal structure of mint leaves and moisture transfer mechanism within them.
500 - (A -a (A ♦
400 - O ♦
200 - /* ♦ \ ♦
100 - ♦ \
n temps(h)
0 9h 10h 11 h 12h 13h 14h 15h 16h
(a) (b)
Fig.3 Energy used for drying Q as (a 1st day, b 2nd day )
The dependence of exergy with drying time can be observed in the curves shown below. The evolution of exergy (input, output) and exergy loss are represented in figure 4 (a,b) for the two days of drying time. We notice a significant increase in these values in the first 4 hours of the first day with a peak at 13H local time, which corresponds to the maximum of solar irradiation. The curves keep the same pace for the two days of drying. The maximum value of the exergy inflow to the system was obtained as 0.120 kJ/kg the first day and 0.105 kJ/kg the second day.
Figure 4 Evolution of exergy as function of drying time(a 1st day, b 2nd day )
In the figure 5 (a,b), the exergy loss evolution according to the energy used Qas is given for two days of drying time . It may be noted that Exl varies linearly with Qas. The exergy losses ranged between o kJ/kg and o.125 kJ/kg approximately during the first day, and between o kJ/kg and o.o9 kJ/kg during the second. As it can be seen, the exergy loss shows a linear increase as the energy utilization of the drying chamber is also increased. Most of the exergy losses were developed during the solar drying of mint performed in the second day.
150 200
Qas(J/s)
150 200 250 300 350 400 Qas(J/S)
Fig.5 Exergy loss E xl according to energy used Qas (a 1st day, b 2nd day )
450 500 (b)
The curves representing the change in the exergy efficiency as a function of drying time are shown in figure 6 (a,b). A decreasing pattern was described during the first 4.5 h, then changing to an increasing behaviour following a parabolic function. Similar behaviour is observed the two days of the drying time.
0,08 -
™ 0,06-
0,02 -
Fig. 6 Exergetic efficiency according to time (a 1st day, b 2nd day )
5. Conclusion
In this work, we had obtained the results of the energy and exergy analysis of the mint drying process in an indirect solar drier operating in natural convection and in a discontinuous way (only the day).
The mint drying samples lasted two days (14 h). The device operates slowly, not exceeding (0.2m/s). By using the 1 st and 2 2nd principles of thermodynamics, we could consider energy useful received by the collector and that is really used during drying. We also determined the efficiency of the solar air heater as well as the energy needed for the drying of mint.
The exergy analysis enabled us to have an estimation of the really supplied energy for drying as well as losses of exergy. The results obtained seem acceptable to us and in agreement with the results found in literature. The insufficiencies observed are in our opinion due to low recorded flows, It appears necessary to repeat these experiments and measures using an air dryer in forced convection.
References
[1] Duffie, Beckman Solar Engineering of Thermal Process , Ed.1980 WIB.
[2] Boulemtafes, A.,Semmar,D. Conception et expérimentation d'un séchoir solaire indirect Revue des Energies Renouvelables, Valorisation Tlemcen, 23-24 Novembre 1999, Tome 1 P. 97-100.
[3] A.Boulemtafes, N.Benaouda Le séchage Solaire des Plantes Médicinales et Aromatiques- Application au séchage de la menthe. Proceeding du 1er Séminaire Maghrébin sur les Sciences et Technologies de Séchage SMSTS'2006,17-19 Décembre 2006 Tozeur, Tunisie.
[4] A.Hepbasli A key review on exergetic analysis and assessment of renewable energy resources for a sustainable future ,Renewable and Sustainable Energy Reviews, Volume 12, Issue 3, April 2008.
[5] AR.Celma Energy and exergy analyses of OMW solar drying process, Renewable Energy Volume 34, Issue 3, Pages 660-666 (2009).
[6] A.Midilli et al, Energy and exergy analysis of solar drying process of pistachio, ENERGY, 28(2003)
[7] S.Janjai et al., Performance of solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices Renewable Energy 30 (2005)
[8] D.Ivanova et al., Energy and economic effectiveness of a fruit and vegetables dryer, Energy Conversion and Management, Vol 44(2003).
[9] C. Tiris et al., Investigation of the thermal efficiencies of a solar dryer, Energy Conversion and Management, Volume 36, Issue 3, March 1995, Pages 205-212.
[10] Turhan Koyuncu, Performances of various design of solar air heaters for crop drying applications, Renewable Energy, 31(2006).
