Drying is one of the most energy-intensive processes in agro-products industry. For this reason, using solar energy appears as an attractive not polluting alternative to be used in drying processes. However, the daily and seasonal fluctuations in the radiation level require using energy accumulators with phase change materials (paraffin wax), to have a continuous drying processes. In hybrid solar dryers with energy accumulation system, a control system is essential to coordinate the control valves that allow the income of air that comes from the solar panel or from the energy accumulator. In this work, we implemented an advances multivariable control system that uses fuzzy logic in the hybrid solar dryer. The dryer includes an energy accumulator panel with paraffin wax as phase change material. The input variables were ambient temperature and solar radiation, both not controllable. The controlled variables were the opening level of the solar panel and accumulator energy valves. The control program consisted in an algorithm implemented with the “Fuzzy” toolbox in Matlab. Data were acquired with OPTO 22. The control system performed adequately when used to dehydrate mushroom slices and plums. Closing or opening the respective valves as a response to the variations of solar radiation and ambient air temperature allowed optimizing the use of solar energy.
The main goal of drying of foodstuff is to reduce the moisture content of the solid up to a level where microbial growth and enzymatic reactions are minimum. Agro-products represent a significant part of the seasonal crops. In order to extend their shelf life, drying is a mayor technology; however, it implies high energy consumption [
Solar drying is the use of solar radiation as unique or partial energy source, in a drying process. Since ancient time, humanity used solar radiation to dehydrate and preserve food, initially by exposing directly the products to the sun. Using solar energy reduces the energy cost and additionally the CO2 emission. In solar drying, the dry- ing rate depends on non controllable external factors such as solar radiation, ambient air temperature, wind speed and relative humidity of the air. Besides it depends also on the substrate characteristics such as initial moisture content, physical properties and surface exposed to the drying air [
Solar dryers have lower operation costs with respect to conventional dryers, being an economically feasible alternative to conventional drying systems [
Optimization of the cost associated to the usage of solar dryers requires an analysis of the local sola radiation, temperature and air relative humidity. The optimal drying period is of 8 hours for drying temperatures between 30˚C and 70˚C [
Smitabhindu [
Thermal energy storage allows using solar energy in low or null radiation periods. This energy accumulation can be performed by storing sensible and/or phase change heat. Phase change heat present advantages due to its high heat density and a minimum temperature variation during charge and discharge periods [
Phase change materials (PCM) receive great attention in recent years for using them to store solar energy for industrial and household applications.
The latent heat of the accumulation system stores energy during fusion and delivers it during solidification of PCM. These materials are classified in organic and inorganic PCM. Organic PCM have the advantage of keep- ing their properties independently of how many times they melt or solidify [
In recent years several publications inform the use of PCM in solar equipments for drying of agroproducts [
The choice of the most appropriate PCM should consider the cost, thermal conductivity (in liquid and solid phase), the energy storage capacity and the phase change temperature [
In order to enhance thermal conductivity of paraffin wax, the encapsulation of PCM has been studied using different geometries being spherical the most promising [
Classic automatic systems operate according to logic of fixed values, and do not allow representing intermediate values which are commonly found in real processes. Fuzzy logic theory permits automating the control of real processes through computer algorithms. In this work, an automatic system to control valves was implemented based on fuzzy logic theory.
Control strategies based on fuzzy logic is currently in development. Levente and Hungerbuhler [
Atthajariyakul and Leephakpreeda [
The hybrid solar dryer (HSD) (
The solar energy accumulator (
The valve (6) regulates the air flow from the solar panel and valve (7) regulates the flow from the solar energy accumulator.
OPTO22 allow monitoring data in real time by the PAC Display Configurator, which develops a graphical in- ter-phase [
Hybrid-solar dryer. 1) Chamber drying; 2) Centrifugal fan; 3) Vent valve; 4) Vent; 5)Recirculation valve; 6) Panel valve; 7) Accu- mulator valve; 8) Solar energy accumulator; 9) Solar panel; 10) Air fresh inlet
Solar panel
Drying chamber with mushrooms
Solar energy accumulator
The control system regulates the air flow rate from the solar panel and form the energy accumulator. When radi- ation level and ambient temperature are high, the drying air is heated only by the solar panel; while simulta- neously the PCM in the energy accumulator melts, thus storing the energy. As radiation decreases, the energy input from the solar panel is insufficient to heat the drying air. Then it is necessary to use the accumulated ener- gy. To accomplish this, the fresh air passes through the accumulator [
The control program consisted in an algorithm implemented with the “Fuzzy” toolbox in Matlab™. The ne- cessary rules were based on previous experimental data obtained with the hybrid solar dryer.
Among the membership functions offered by Matlab we selected trapezoidal (trapmf), Gaussian (gaussmf) and triangular (trimf). The membership function and the parameters of each input and output variable were se- lected based on empirical knowledge of the hybrid solar dryer.
Each variable has its own membership function. Radiation is an input variable that fluctuates between 0 and 10000 (0 - 1200 (W/m2)). The parameters are given in
The ambient temperature is an input variable and fluctuates between 0˚C and 40˚C. The parameters are given in
Panel valve and accumulator valve are output variables that fluctuate between 0 and 100. The parameters are given in
. Rules for the fuzzy logic controller
Radiation | Air temperature | Panel valve | Accumulator valve |
---|---|---|---|
High | High | High | Low |
High | Medium | High | Low |
High | Low | High | Low |
Medium | High | High | Low |
Medium | Medium | High | Low |
Medium | Low | High | Low |
Low | High | High | Low |
Low | Medium | Medium | Medium |
Low | Low | Low | High |
. Membership function for radiation
Level | Type | Parameters |
---|---|---|
Low | trapmf | [−200; −100; 200; 000]">3000] |
Medium | gaussmf | [1000; 000]">4000] |
High | trapmf | [5000; 7000; 20,000; 25,000] |
. Membership function for ambient temperature
Level | Type | Parameters |
---|---|---|
Low | trapmf | [−10; −5; 20; 25] |
Medium | trimf | [22; 25; 28] |
High | trapmf | [25; 30; 45; 50] |
In the first 8 hours the solar panel valve was completely open, while the accumulator valve was completely closed. In this period the paraffin wax melted owing to the solar energy absorption. From the 8th hour until the 10th hour the panel valve begins to close and the accumulator valve opens gradually. In this period the stored energy is withdrawn by the drying air. Then the drying air temperature equals ambient temperature.
. Membership function for panel valve and accumulator valve
Level | Type | Parameters |
---|---|---|
Low | trimf | [−40; 0; 20] |
Medium | trimf | [10; 50; 90] |
High | trimf | [80; 100; 140] |
Fuzzy logic control system
Behavior of the accumulator and panel valves in a sunny day
The opening of the valves followed a trend according to solar radiation level. After the first hour of drying, solar radiation decreased sharply, observing a notorious change in valve opening. From the 6th hour onwards radiation decreased up to 300 (W/m2) and again the valves change their opening level.
In order to evaluate the performance of the hybrid solar dryer using the fuzzy logic control system, 8-mm mu- shroom slices were dehydrated using only solar energy. Initial load was 15 (kg) and the drying period was 11 hours, diminishing the moisture content from 93% to 6% (
The fuzzy logic controller adequately controlled air flow rate (
Behavior of the accumulator and panel valves in a partially cloudy day
Mushrooms. (a) Fresh; (b) Dehydrated
Behavior of the accumulator and panel valves during mu- shroom drying
Temperature in the outlet of the solar panel and energy accumulator, during mushroom drying
Drying kinetics of mushroom slices, using control system
Charges of 10 kg plums were dehydrated using only solar energy to heat drying air. The total drying period was 10 hours, coinciding the first 7 hours with the highest solar radiation period, and after that (the last 3 hours) the energy was taken from the solar accumulator. The initial moisture content of plums was 80%, reaching a 71% after 10 hours of drying. Plum skin avoided a higher moisture loss in this period. Since the final moisture is too high for preservation, the drying process should continue in the following days.
As shown in
The fluctuations of the air temperature at the outlet of the solar panel (
Behavior of the accumulator and panel valves during plum drying
Air temperature in the outlet of the solar panel and energy accumulator during plum drying
Drying kinetics of plum
Comparing mushroom (
We implemented an advanced control system based on fuzzy logic in a hybrid solar dryer to control the opening of solar panel valve and energy accumulator valve in function of solar radiation and ambient temperature.
The fuzzy logic controller performed adequately during the drying of mushroom slices. This resulted in an in- crease of thermal efficiency of the process. During 11 hours of drying, the moisture content of slices mushroom diminished from 93% to 6%. During 10 hours of drying, the moisture content of plum diminished from 80% to 71%.
The communication system based on OPC technology works in a very good way for programming fuzzy logic control implemented in Matlab with the data acquisition program OPTO22.
The authors thank the financial support of CONICYT through grant Fondecyt 1110101.