The fluctuation in drying temperature influences the food products’ quality and drying time significantly during the drying process using an indirect solar dryer. One of the effective methods to reduce these variations in the temperature is based on thermal storage materials to control the drying temperature. An experimental investigation is presented in this study to evaluate the performance of an indirect solar dryer with air double pass using multiple phase change materials (PCM) as thermal storage materials. Two PCMs with different melting points are used to store the available heat energy during peak sunshine periods and reduce the drying temperature fluctuations. Drying tests on a food product sample are conducted in three cases, the first was without PCMs using natural convection. The second and third cases are based on forced convection with and without using multiple PCMs under Baghdad city conditions. The results showed that; approximately a steady temperature of hot drying air was obtained during relatively low ambient temperatures when the dryer was incorporated with multiple PCMs. The drying time of the product sample for the natural, forced convection without PCMs and forced with PCMs was 87, 72 and 47 h, respectively. The higher performance of the dryer was for the forced convection with PCMs. The reduction percentage in drying time was about 17.2% and 46% for the forced convection without and with PCMs respectively compared with natural convection. The average drying rate for the natural, forced without PCMs and forced convection with PCMs was 0.0093, 0.0135 and 0.0172 kg/h, respectively. The enhancement in thermal efficiency of the solar collector and drying chamber with multiple PCMs was 43% and 25.5%, respectively, compared with a typical solar dryer.
Drying of food products using solar energy as an environmentally friendly and sustainable energy source represents an effective and economical method to preserve the food products for a sustainable world. Sustainable energy technologies such as solar drying can significantly contribute to developing the processing systems of agricultural products to enhance economic growth, particularly in developing countries. Therefore, continuous research activities are required to develop the solar drying equipment, specifically direct and indirect dryers. The indirect solar dryer’s thermal performance can be feasibly improved if the dryer is incorporated with thermal storage material such as phase change materials (PCM). Using single or multiple PCMs with different melting temperatures will feasibly utilize the solar energy and at the same time prevents the fluctuations in temperature of the drying chamber that occurred in case of using a conventional solar dryer. In the extreme hot region like Baghdad city in Iraq (Latitude 33° 19′ N and longitude 44° 25′ E), where the solar radiation is available during relatively long light hours, there is a large difference between the ambient temperatures through the sunlight period. Therefore, during peak sun hours, the available heat energy could be lost if it is not stored using a heat storage device. Additionally, the variation in drying temperatures resulting from weather fluctuations will affect the drying quality of the food products. These disadvantages can be reduced by using multiple PCMs. The previous relative studies on solar air collectors and thermal systems without and with PCMs were reviewed by many researchers [
Two phase change materials (PCM) of different thermophysical properties are used to improve the performance of an indirect solar dyer with air double pass. The first material (PCM1) is used to store the excess heat energy through the peak sunshine hours, while the second material (PCM2) is utilized to reduce the fluctuations in temperature of the drying chamber and keep it at the desired point. The thermophysical properties of PCM1 and PCM2 are illustrated in the
Properties | PCM1 (Paraffin wax) | PCM2 (RT-42) |
---|---|---|
Melting temperature |
57°C |
38–43°C |
Referring to
The rate of heat transfer (
where the values of
Overall heat transfer coefficient (U) of the flat plate solar collector can be expressed by:
where the rate of heat (
Mass (
Discharging time (td1) of PCM1 during solidification process through a relatively low solar radiation period can be estimated by:
The rate of heat transfer (
where
where
Charging time (
The mass (
Discharging time (
The mass of water (
where
The moisture ratio (
where:
The drying rate (
Thermal efficiency of the air double pass indirect solar collector (
Thermal efficiency of the drying chamber (
An indirect solar dryer with air double pass using multiple PCMs based on forced convection was used to conduct the experimental work in the present study. The solar dryer rig is fabricated during the current study, consisting of two main parts, solar collector for heating the air and drying chamber for drying the food product sample, as shown in
A centrifugal air fan of 25W capacity, model AKS 680–180 with power regulator, is used to supply the air with flowrate 0.005–0.012 kg/s in the solar dryer. The air fan is installed under the solar collector to circulate the air around the PCM1 container and over the absorber plate for heating the air before entering into the drying chamber, as shown in
Equipment | Accuracy |
---|---|
Temperature data logger (BTM-4208SD) |
±0.2°C |
Experimental work is conducted in November 2020 through sunshine hours from 9 am to 5 pm under Baghdad city conditions (Latitude 33° 19′ N and longitude 44° 25′ E). Variations of ambient temperature and solar irradiance with day time are illustrated in
The performance of the drying process depends on a reduction in moisture content of the food product sample (chili pieces) during sunshine hours. The initial moisture content of the sample was 87% and gradually reduced with time at various ranges to reach about 15% as a final desired content of the moisture for the product sample, as shown in
The variations of drying rate with the moisture ratio of the product sample is depicted in
Comparing the efficiency of the drying chamber with and without PCM during drying time is shown in
An indirect solar dryer’s thermal performance with and without multiple PCMs as thermal storage materials has been investigated in the current study. Drying tests are conducted in three cases, the first case was without PCMs based on natural convection as baseline case for comparison, while the second and third cases are based on forced convection with and without multiple PCMs. Steady air outlet temperatures from the solar collector and drying chamber are obtained during relatively low ambient temperatures and irradiance after the hour 14 of day time due to the effect of multiple PCMs. The drying time of the product sample at the three cases, natural, forced without PCMs and forced with PCMs, was 87, 72 and 47 h, respectively. Higher performance of the dryer was for the case of forced convection with PCMs. The reduction percentage in drying time was about 17.2% and 46% for the cases of forced without and with PCMs respectively compared to natural convection. The enhancement in the efficiency of solar collector and drying chamber with PCM was about 43% and 25.5% respectively compared to that without PCM. For the three cases, natural, forced without PCMs and forced with PCMs, the average drying rate was 0.0093, 0.0135 and 0.0172 kg/h, respectively.
area of the absorber plate (m2)
air specific heat (J/kg.°C)
heat removal factor for the collector
water latent heat of evaporation (J/kg)
solar irradiance (W/m2)
thermal conductivity (W/m.°C)
latent heat of fusion of PCM1 (J/kg)
latent heat of fusion of PCM2 (J/kg)
air mass flow rate (kg/s)
drying rate of the product sample (kg/h)
mass of the food product sample (kg)
initial mass of the food product sample (kg)
final mass of the food product sample after drying (kg)
mass of PCM1 (kg)
mass of PCM2 (kg)
moisture content on a dry basis (%)
moisture content on a wet basis (%)
mass of the water removed from the product sample (kg)
rate of heat transfer to the first PCM (W)
rate of heat transfer to the second PCM (W)
rate of heat transfer to the air (W)
ambient temperature (°C)
temperature of inlet air to the collector channel (°C)
temperature of the outlet air from the collector channel (°C)
temperature of the collector absorber plate (°C)
temperature of PCM1 (°C)
temperature of PCM2 (°C)
wet bulb temperature (°C)
charging time of PCM1 (s)
charging time of PCM2 (s)
drying time (s)
discharging time of PCM1 (s)
discharging time of PCM2 (s)
overall heat transfer coefficient (W/m2. °C)
absorptivity of the absorber plate
transmissivity of the collector glass cover
thermal efficiency of the indirect solar collector (%)
thermal efficiency of the drying chamber (%)
first phase change material
second phase change material