Analyses of thin layer drying characteristics of mint leaves in a forced convection solar dryer in terms of energy and exergy
This paper addresses the energy and exergy analyses carried out after the process of thin layer drying applied on the mint leaves using a forced convection solar dryer system. The energy analysis was conducted in accordance with the first law of thermodynamics in order to predict the energy usage rates and the energy amount generated by the solar air collector. Nevertheless, the exergy analysis was carried out in accordance with the second law of thermodynamics in order to find out the exergy losses during the drying process. The drying experiments were carried out between the mass flow rates of 0.012 kg/sn and 0.033 kg/sn at three different drying stages. The effects of inlet air velocity and drying duration both on energy and exergy was also investigated. As the result, it was concluded that both energy usage ratio and exergy loss have decreased, while causing an increase in drying mass flow rate as the exergetic productivity has increased.
Keywords : Energy analysis, Exergy analysis, Thin layer drying, Forced solar dryer, Mint.
It is widely accepted that mint is a plant that is harvested by laying a sheet on the ground, because of containing a high level of moisture inside. Due to short season and as the plant is vulnerable against storage, drying is a method that is oftenly used in order to provide conservation 1. Dried mint is generally used in food and beverages; and spread over salads. Drying process is described as taking the moisture away from the substances. The drying process is regarded as one of the most significant processes in the conservation of the agricultural goods, since the dried crops’ properties are highly affected by the method used in the drying proess. It has long been known that fruit and vegatable drying is one of the most commonly used methods in the conservation of the food products. What is intended in drying agricultural crops is to reduce the moisture content in them, which allows them to be safely stored for longer periods. Drying the crops via solar means is most commonly used method both in Turkey and all over the world to establish convenient preservation conditions for agricultural crops.
Nevertheles, the process mentioned above has a few problems such as contamination of the crops with dust, soil, sand particles and even insects2.
Though solar drying carry a great amount of disadvantages, it is still widely used in many parts of the world. Since it is a cheaper type of energy source possessing the properties such as being ample, limitless, renewable, cheap and not-polluting the environment, the energy obtained from the sun has been considered to be a very important alternative energy. Recently, a number of independent studies concerning the mathematical modelling and finding the drying kinetics regarding theproducts such as vegetables, fruits and agro-based goods were carried out by numerous researchers. Among these, there were studies carried out on pistacia vera 3,22, carrot4, apple and potato 5, red pepper 6, fig 7, crop 8, mulberry 2,9,10, mint leaves 11,27, hazelnut 12, grapes 13,24,25,26, apricot 14, and silk cocoon 23.
The fact that Mathematical models are extremely useful in designing and analysing the processes such as heat and mass transfer during the drying period have been proved by previous studies.
The thermodynamic analysis, particularly the analysis of the exergy, has been considered as a requirement for the optimization of a thermal system as well as for its analysis and design15. Exergy is the maximum amount of work that is producible by a stream of matter, heat or work in terms of the equilibrium to be reached when compared to an environment that is considered as the reference 16. In the process of drying the objective is to use the least amount of energy for the highest level of moisture removal, which is desired in final conditions of crop. Up until now, a large number of investigations have been carried out on carrying out the analyses of the exergy produced while dryingfood. However, the literature review, which was carried out extensively within the scope of the present study, shows that no subtle information on energy and exergy analyses on the subject of thin layer drying process of mint using forced convecton solar dryer have been found. For this reason, this paper, since it is different from the studies in the literature, focuses on energy and exergy analyses of the thin layer drying process applied on mint using forced solar type dryer by employing the thermodynamics’ first and second laws. It is regarded that a study of this kind is going to have an enormous contribution to the aplications of the mint producers, since it removes their problems about energy and exergy during the overall drying process. The main objective of the study is to present the results of the analyses of energy and exergy generation after drying the thin layers of mint with various drying mass flow rates in a forced solar dryer. Similar to many other significant studies on the drying process carried out by analysing the generated energy and exergy, the study below may also be mentioned. Midilli and Kucuk 17, carried out the analyses of energy and exergy production of the drying processes of pistachios within and outside of the shells, using a solar drying cabinet. Furthermore, Dincer and Sahin 15 have developed a neuvel model for the analysis of drying process by means of thermodynamic principles. Akpinar18 have used convective type dryer in order to carry out energy and exergy analyses of the drying process of red pepper slices.
The exergy analysis of the thin layer drying of green olive was performed by Colak and Hepbasli 19, using a tray dryer. Akpinar et al. 20 analysed the pumpkin’s drying process using the thermodynamic’s first and second laws.
Corzo et al. 21 anlysed the generated energy and exergy during thin layer drying of coroba(corona olabilir mi?) slices, using three different air temperatures,.
Material and methods
Because of its geographical situation, being located in the Mediterranean Region (Between the Northern latitudes no. 36º and 42º), Turkey has an abundant solar energy potential. In Turkey, the period that the sunshine can be used is 2624 h/year. The maximum duration for this time period is in July as 365 h/month, and the minimum is 103 h/month, which occurs in December. The average intensity of the solar radiation is approximately 3.67 kWh/m2 per day.
In this study, the solar cabinet dryer was mounted inside the garden of Firat University, Technical Education Faculty, Elaz??, Turkey. The experiments on this solar drying system were conducted starting from August until September 2005. Starting at 09:00 am, each test was carried out until 17:00 pm. In this process, mint drying was performed in a solar cabinet dryer.
The diagram and a photo of this solar dryer system are presented respectively in Fig. 1 a and b. This system basically consists of four subsystems; which are (a) drying cabinet, (b) solar air collector, (c) air fan and AC hertz converter (d) data logger. By using the intervals of 15 min., the data regarding the air temperature, inlet and outlet temperatures of the solar collector and dryer, temperature in the centre of the mint , the value of the relative humidity that is taken right above the bed surface of the mint and the solar radiation were recorded during the experiments. Using the ZA9000FST connector element, T Type copper-constant thermocouples were connected to 5990-0 Almemo digital data logger, which has a reading accuracy of ± 0.1ºC, for measuring the temperature within the system. For measuring the air speed in the system within a range of 0.1-15 m/s, the thermo anemometer (FVA645TH3) was used. By using the FDA612MR pressure module, the decrease in the pressure withinin the collector was calculated. The mass that the mint lose was recorded until the end of the drying process, in order to determine the drying curves with an accuracy of 0.01 kN using FKA0251 strain strength in the range of 0.02-10 kN. During the drying process, the solar radiation was computed using the solarimeter developed by Kipp and Zonen. The fresh mint used in the study was purchased from a local market in Elaz??, Turkey. By interfacing the Almemo 5990-0 data logger to a personal computer, all the data was collected; afterwards, they were recorded at time intervals of 15 min. In order to calibrate the system, the drying system ran for at least 60 min. before the sample was placed in the dryer,. A centrifugal fan was utilized in the solar dryer system in order to blow air into the solar collector through a flexible aluminium duct having a diameter of 82 mm. Since an AC type hertz converter was used, the air flow mass could be controlled. The inner chamber of the dryer unit was produced using a 0.8 mm thick stainless steel sheet in the dimensions of 1.2 x 0.74 x 0.74 m, which was respectively enclosed in an outer chamber in the dimensions of 1.5 x 0.75 x 0.75 m, again produced using stainless steel sheet. Polystyrene insulating materials were filled in the space between the two chambers for a quality insulating.
The airpassed through the chamber of chimney with the dimensions of 0.3 x 0.2 x 0.2 mafter leaving the heating chamber, and this process ensured its being mixed and having a uniform temperature distributon before going into the drying area. During experiments, fresh mint, having0.3 kg of water/kg of dry solids as the average initial moisture content was used in the dryer. No treatment was carried out on fresh mint before the experiments.
As the result of the analyses carried out within the scope of the thermodynamics’ first and second laws, it was considered that there was a steady flow type drying process. The thermodynamics of the damp air were used as the basic points for these analyses.
3.1. The analysis according to first law of thermodynamics
In order to determine excessive information about energy and drying air behaviour using a forced solar dryer, an energy analysis of the thin layer drying process of mint was carried out within the context of the thermodynamics’ first law, ,. The air conditioning process from the beginning till the end of the mint drying comprises heating, cooling and humidification processes. In reality, this process may be defined as a steady flow process which is analysed by using the principles of conservation of mass and energy in a steady flow state.
The equations presented below are generally used to calculate the mass conservation of the drying air and moisture, the energy conservation of the process and relative humidity and enthalpy of the drying air for analysing the the energy and exergy generation within a thin layer drying process, ,.
Mass conservation of the drying air is calculated using the equation of:
Mass conservation of the moisture is calculated using the equation of:
Conservation of the energy is calculated using the:
Only the alterations in the kinetic energy levels inside the fan were considered, while the potential and kinetic energy changes were neglected for the remainder of the system:
Where P is the atmospheric pressure, w shows the particular humidity, , [email protected] is the saturated vapour pressure for the drying air.
Enthalpy within the drying air can be established as below:
In order to determine the solar collector’s outlet conditions, it was presumed that there was no heat loss from the beginning until the end of the pipe used for the connection between the fan and the solar collector, and hence, the solar collector’s inlet conditions are almost equal to the fan’s outlet conditionsas stated in equations (7):
The energy conveyed from the solar collector to the drying air may be calculated by both using outlet and inlet temperature values of the solar collector using the following equation:
Obviously, certain losses of heat will occur between the outlet of the solar collector and the inlet of the dryer, when the temperature values are taken. It should certainly be stressed that the conditions of the solar collector outlet are not equal to conditions of the dryer inlet, in accordance with the heat losses that take place in this part of the system,.
Therefore, in the following equation, the prediction of the amount of losses in the heat values across the pipe used for the connection of the solar collector and the dryer is presented:
The equation presented below and the psychrometric chart provides the used heat during the dehumidification process in drying chamber:
The drying chamber’s Energy utilization ratio (EUR) may be calculated during this drying process by using the equation below:
3.2. The analysis according to second law of thermodynamics
The overall inflow, outflow and losses of the exergy in the forced solar dryer system were predicted within the context of the analysis according to second law of thermodynamics, . The fundamental procedure that is to be followed in the analysis of the drying chamber’s exergy generation,is to determine the exergy generation values at the points of steady state, and to find the reasons for the alterations of exergy generation during the process. The exergy production values can be calculated by making use of the working medium’s characteristics of the enegy balance according to the first law.. To this end, the exergy equation, which is generally applied or the systems with steady flow statuses, is as follows 17.
Where the the reference conditions are denoted by the subscript ?. While there are different types of this equation for calculating the exergy values in general, some, but not all, of the terms shown in Eq. (12) are utilized in the analyses of many systems. As exergy is basically the energy that is easy to obtain from any kind of source, the electrical current flow, the magnetic fields, and the diffusional flow of materials can be used to build up certain terms. To simplify in a general manner, replacing the enthalpy with the terms of internal energy and PV, which are feasible for the systems that have steady flow rates. Eq. (12) has frequently been used when the momentum and the gravitational terms are neglected. Furthermore, the alterations in the pressure values within the system should also be neglected because of the fact V=V?.
When that type of a case is the matter, Eq.(12) will be derived as below:
Putting Eq.(13) into action, the exergy’s inflow and outflow may be determined based on the drying chamber’s inlet and outlet temperatures. Then, by employing Eq.(14), the exergy loss values throughout the drying process are determined.
The equation that computes the exergy inflow to the drying chamber is:
Where cpda is the drying air’s average specific heat. Nevertheless, the exergy outflow equation may be stated as;
Eventually, the amount of the loss in the exergy will be computed using the Eq.(14). The exergetic efficiency can be described as the ratio of the exergy which is used in drying the product exergy to exergy inflow for the drying chamber. However, this case is made clear as the ratio of the exergy outflow to the exergy inflow for the drying chamber(Bu cümlelerde biraz kafam kar??t?, o yüzden yanl?? bir?ey yapmamak ad?na dokunmad?m, bilgilerinize). The drying chamber’s exergetic efficiency may be predicted by taking these definitons into consideration. Therefore, the exergetic efficiency’s commonly used form has been scripted down as shown in the following equation17;
4. Results and discussion
It was summer season, from August to September 2005, when drying experiments were carried out in the province of Elaz??, Turkey. The alteration of solar radiation was recorded to be between 125 W/m2 and 750 W/m2 while the maximum temperature time was recorded as 11.00 and 15.00 during the experiments of thin layer mint drying using forced solar dryer,. The maximum solar radiation energy was found to be at midday, while its minimum value was in the evening hours on the day of the experiment. Using data from the results of the experiments and values obtained out of these computing that were demonstrated in Figs. 3-4-5-6-7-8, and argued in detail, the analyses of energy and exergy generation during the thin layer drying process of mint using forced solar dryer were carried out.
4.1. The content of moisture
The variations in the moisture content for the mass flow rates of 0.012 kg/s, 0,026 kg/s and 0,033 kg/s are shown in Fig. 2, as a function of the duration of drying. As the mint’s moisture content decreases during the process in the solar type dryer, the moisture diffusion from the mint into the air will also decrase. It is also possible to observe that the drying air’s relative humidity will decrease in a parallel way to the moisture content inside the mint.
4.2. Analysis of energy utilization
The analysis of the energy production during the thin layer drying process of mint was performed using the data that were obtained as the result of the experiments performed using forced solar dryer,. Figs. 3-4-5 display the results of the analysis of the energy production during the thin layer drying process of mint using forced solar dryer. Through the Eq.(10), the values of energy utilization in the drying chamber were calculated. EUR, which was found by using Eq.(11), has been identified as the ratio of the energy utilization to the energy that was provided by the solar collector. The upmost values of the Qcol and Qdc were obtained as 445.6 W and 344.7 W, where the mass flow rates were determined as 0.012 kg/s during the 480 minute experiment procedure, respectively.
At the same time, the Fig. 3 demonstrates the alterations in the energy utilization ratio (EUR) for the drying process carried out with the mass flow rate of 0.012 kg/s. It was observed that the EUR values presented differences between the rates of 10.4% and 87.1% during the experiments that were performed. Fig. 4 displays the values of Qcol, Qdc and EUR for for the mass flow rate of 0.026 kg/s for the drying air. The maximum values of Qcol and Qdc were found out as 623.4 W and 416.5 W, where the mass flow rate was 0.026 kg/s during 390 minute experiments, respectively.
However, Fig. 4 exhibits the variations of the EUR ranging from 12.5% to 54.75% in drying chamber during these experiments.
Fig. 5 shows the results of the analysis carried out on energy production and utilization during the drying process for the mass rate of 0.033 kg/s. It was found out that Qcol and EUR ranging from 326.22 W to 705.82 W, 7.9 % and 33.66%, respectively. Upmost value of Qdc was obtained as 382,34 W, where the mass flow rate was 0.033.
The rates of % 87.1, %54.75, %33.66, respectively, were found to be the average values of EUR for the 0.012, 0.026 and 0.033 mass flow rates of the drying air.. These values demonstrate that the value of the drying chamber’s EUR decreased along with the increase in the mass flow rate of the drying air.
4.3. Analysis of exergy utilization
The analysis of the exergy production during the thin layer drying process of mint using forced convection solar dryer was conducted by making use of the data that were obtained through the experiments on the drying processes. The values of ExL and ?Ex for the drying air’s each mass flow ratio will be seen in Figs. 6-7-8. During the experiments using three different mass flow rates, the exergy loss that was calculated inside the drying chamber increased during the very first 300 min., and subsequently, it displayed a decreasing behaviour. It is clear that, this time variation resulting from the exergy loss occurred as a result of the alterations in the solar radiation.
The average values of the ExL for the mass flow rates of 0.012, 0.026 and 0.033 kg/s for the drying air were calculated as 16.22 W, 8.2 W, 6.88 W, respectively. The maximum exergy loss was determined at the time that the mass flow rate was 0.012 kg/s. The lowest exergy value was calculated when the mass flow rate was 0.033 kg/s. The values that were determined demonstrate that the exergy loss decreased together with the increase in the drying air’s mass flow rate. Moreover, it can also be stated that the amount of the change in the radiation has influenced the exergy loss in the system. Furthermore, the exergetic efficiency values for the drying chamber are also displayed in Figs. 6-7-8. For the drying air’s each mass flow rate, the exergetic efficiency values depending on the inflow, outflow and loss of exergy was computed by using the Eq. (15), . The drying chamber’s exergetic efficiency has increased with the decrease in the temperature difference between the dryer chamber’s inlet and outlet.
The exergetic efficiency values were determined to be 33.88 – %75.6 for the mass flow rate of 0.012 kg/s. Nevertheless, for the drying air’s mass flow rate of 0.026 kg/s, the exergetic efficiency changed from %46.44 to %71.22. For the mass flow rate of 0.033 kg/s of drying air, the exergetic efficiencies displayed a difference between %40.1 and % 68.23. These obtained values demonstrate that the drying chamber’s exergetic efficiency decreased when the energy provided by the solar collector was used in an effective way.
The impacts of the connective solar dryer on the drying process of the mint at three different mass flow rates was investigated one after the other. The duration of the drying process decreased considerably at the time that the mass flow rate increased.
The drying process came in existence in failing rate period (Bu cümleyi de anlayamad?m). Within the framework of this study, the analyses on the energy and exergy production and utilization for the thin layer drying process of mint were conducted using a forced solar type dryer. When the outcomes from these analyses are taken into consideration, the statements presented below may be expressed:
The mint samples were dried adequately till final moisture content of nearly 0.08 kgwater / kgdry matter was obtained at the ranges from 0.012 to 0.033 drying air mass flow rates during 300-480 min. and under 125 W/m2- 750 W/m2 solar radiation.
It is possible to state that the energy received from the solar collector rose with the increase of the mass flow rate of drying air.
The energy received from the solar collector was efficiently utilized for drying chamber when the energy utilization ratio (EUR) went up. As a crucial note, it is stated that the energy utilization ratio would be presumed as a significant parameter to analyse the utilization of energy in thin layer drying process.
The exergy loss fell down with the rise of the mass flow rate of drying air. Moreover, it will be possible to be said that the value of radiation influenced the exergy loss. Great number of the exergy losses occurred for the 0.012 kg/s mass flow rate.
So as to lower the energy utilization in drying chamber, an optimization investigation must be conducted leading for the betterment of collector productivity using various hindrances in the air flow duct to increase the heat transfer area.
As a result, it is recommended that the layout, structure and moisture content of the products in the drying chamber be taken into consideration to lower the energy utilization and exergy losses.
It is essential to display the alterations of exergy with drying time to determine when and where the maximum and minimum values of the exergy losses occurred at the time the drying process.
Drying mint in natural environment lasted nearly 930 minutes for zero mass change for natural drying before the process of drying was commenced with the help of solar air collectors. When these data were examined, it is easy to see that drying process lasted about two days. The moisture content of mint is 3.55 maximum. Drier air has a major effect on the flow of moisture content alterations over time: m = 0.012 kg / h for 480 minutes for the moisture content, m = 0.026 kg / s for 390 minutes, and m = 0.033 kg/s is nearly zero to 300 per minute.