This paper introduces a temperature control technique for a medical herb dryer system. The technique fixes the drying temperature of the medical herbs at 40℃ even in cases of rapidly changing atmospheric conditions. The control of the dryer temperature is achieved through using on/off controller. The designed dryer contains two systems, which are the thermal and electrical systems. The thermal system is designed to heat the drying air by using solar water collector and electric heater. While, the electrical system which contains a photovoltaic (PV)-WIND hybrid system is used to feed the different electrical loads of the dryer system. The control technique is investigated through simulation work by using MATLAB-SIMULINK. The simulation results indicate the high capability of the proposed technique in controlling the drying temperature, even in case of rapidly changing atmospheric conditions.
Most agricultural crops, which are intended to be stored before use, have to be dried first. Otherwise insects and fungi, which thrive in moist conditions, render them unusable. Examples include wheat, rice, coffee, herbs, etc. All crops drying involve a transfer of water from the crop to the air around it, and this process can be accomplished by passing a hot and unsaturated air over the wet crop. It is also important to realize that there are limiting temperatures for drying corps for storage, so the product does not crack and allow bacterial attack [
The block diagram of the complete thermal system is illustrated in
1) The solar collector;
2) The storage tank;
3) The heat exchanger.
The solar collector is considered to be the main source of the thermal energy or heating in the system. It is used to directly utilize the incident solar energy from the sun and transfers it into the heat transfer fluid, which is chosen to be the water in this work. Also, the storage tank is used, in this work, to store the thermal energy of the water gained in the collector stage. The output water from the tank, which carries certain amount of thermal energy and at specified temperature, enters the heat exchanger stage. This heat exchanger is utilized to transfer a certain amount of the thermal energy of the hot water to the dryer’s inlet air. In this way, the temperature of the dryer’s inlet air can raise to certain temperature. If this temperature is lower than the required drying temperature, then an auxiliary heating source (the electrical heater) will be utilized to supply the deficit in temperature.
It is known from the previous discussion that the main function of the electrical heater is to supply the auxiliary heating required by the drying air that supplements the thermal energy deficit of the solar thermal system; to adjust the drying air temperature at the desired optimal value (i.e., 40˚C). Therefore, the suggested block that controls the air drying temperature is shown in
On-off control is perhaps the simplest type of feedback control strategy. The strategy is similar to that of a relay. The following equations define the output of on-off controller [
Output = ON if the controller is in the ON state;
Output = OFF if the controller is in the OFF state.
where e is a error and output when ON and output when OFF are values determined based on the application.
When the ON-OFF controller is in the ON state, the controller remains in the ON state until the input to the controller falls below the value of the switch OFF point state until the input exceeds the value of the SWITCH ON point.
The ON-OFF controller is initially in the OFF state. In short, the On-OFF controller output one of two values (“output when ON” or “output when OFF”) based on the value of the input signal.
A system under ON-OFF control will always oscillate. To avoid rapid switching between the ON and OFF states, it is common to introduce a hysteresis (ζ) in the switch. The input-output relation for an ON-OFF controller with hysteresis is shown in below
The main function of the electrical heater is to make the auxiliary heating of drying air. The auxiliary heating means that the electrical heater will operate only if the temperature of the output air THE from the heat exchanger is lower than the required value to adjust the air drying temperature Tdryer at 40˚C.
If the rated power of the electrical heater [
Thus, by applying the energy balance principle on the electrical heating chamber will yield [
where VHC volume of the heating chamber of the electrical heater
The complete components of the considered thermal system are simulated in this work by using MATLABSIMU-LINK [
The purpose of the temperature control system is to adjust the dryer temperature Tdryer at 40˚C, which is a necessary value for drying the medical herbs. Therefore, to investigate the performance of the proposed temperature control system the response of the drying temperature must be indicated at different cases.
The robustness of the used controller in rejecting the sudden variations in the insolation level or in the ambient temperature is shown in Figures 5 and 6, respectively. Where, Figures 5(a) and (b) illustrate the sudden variation in the insolation level at constant ambient temperature of 23˚C and the corresponding response of Tdryer, respectively. Also, Figures 6(a) and (b) indicate the sudden variation in the ambient temperature at constant insolation of 1000 W/m2 and the corresponding response of Tdryer, respectively. Therefore, the used on/off controller is sufficient for controlling this system and also has a good capability in rejecting the imposed sudden variations in the solar insolation or in the ambient temperature.
The meteorological data in this work are obtained at two different days with different atmospheric conditions. Figures 7(a) and (b) show the variations of the solar insolation incident on the surface of the solar collector on summer and winter days (in Sinai Peninsula (Abu Rudies) [8,9]), respectively. While,
The performance of the solar collector during the sum-
mer and winter days is shown in Figures 9 and 10, respectively.
Figures 11(a) and (b) show the performance of the storage water tank during the summer and winter days,
respectively. Thus, the effect of stratification on the tank is shown in this figure, where the bottom layer exhibits the lowest temperature and the top layer exhibits the highest temperature of all the tank layers. It is indicated that the temperatures of the tank layers increase during the daylight hours and decrease during the night hours, such that the variation of the temperatures is proportional to the variation in the solar insolation.
The performance of the heat exchanger during the summer and winter days is shown in Figures 12-15, respectively. Figures 12 and 13 include the switching control signals of the air blower and the heat exchanger side, respectively. While the performance of the inlet and outlet temperatures of the heat exchanger water and the performance of the inlet and outlet temperatures of the heat exchanger air are shown in Figures 14 and 15, respectively.
Thus, it is clear from these figures that the switching control signal of the air blower exists as long as there are herbs in the dryer, and that of the heat exchanger side exists as long as the ambient temperature is lower than the desired drying temperature Tref. Also, the inlet temperature of the heat-exchanger water is the temperature of the tank top layer (
Finally, the performance of the electrical heater is shown in Figures 16 and 17, respectively for the summer
and winter days.
The optimal drying temperature of the medical herbs
must be adjusted to about 40˚C. A control technique is proposed to adjust the dryer inlet temperature at the required optimal value. The proposed control technique is based on using the on/off controller, to control the operation of the electric heater whenever necessary. The electric heater will be active only when the temperature of the outlet hot air from the heat exchanger THE is less than 40˚C. The simulation results indicate that the used on/off controller with the tuning of the deadband gives a good capability in rejection, the imposed sudden variations in the solar insolation or in the ambient temperature. Therefore, the developed herb dryer system will be incorrect and continuous operation during the summer/winter day and night hours.