Exergy of Solar Dryer

Abstract

Due to the fact that it eliminates extra moisture and increases food products’ shelf lives, drying is an energy-intensive process in food preservation. Both renewable and non-renewable energy sources can be used to generate the energy needed for drying. Researchers have recently given sources like solar energy the highest consideration when employing renewable energy. Solar energy is the best source of energy for the drying process with solar dryer systems because it is free, clean, available, and economically viable. The usage of solar dryers in agricultural production areas like farms and gardens conserves a variety of energy resources (such as fossil fuel), improves food-processing efficiency, and lowers the cost of transportation. The main components of solar dryers are the fan, the solar air heater (SAH), and the dryer chamber, which is why there are different exergy factors. In the industry of solar dryers, it is crucial to improve drying energy effectiveness and lower energy consumption costs. Using modern technologies makes it easier to improve energy efficiency and lower operational expenses. The main goal of many studies today is to evaluate the energy costs of various drying techniques. This technique, also known as exergy economic analysis, makes sure that the primary contributing factors to system exergy loss are recognized and understood.

1. Introduction

Most countries in the world are facing the problem of running out of fossil fuels; many scientists are using renewable energy to solve this problem. Therefore, it is very important to use systems that work with renewable energy. One of these methods is the use of solar dryers, which have different types and are capable of drying all kinds of products and medicinal plants, and help the country’s economy a lot. To increase product quality, reduce product waste in gardens and packaging industries, and use dried products in seasons where fresh products are not available, drying with a solar dryer is the best method. A significant portion of the moisture content of the items is removed during the dehumidification process, which also significantly lowers the activity of microorganisms during the storage time. Open sun drying (OSD) is an easy method with a very low cost. With this method, most agricultural products can be dried and it is an effective method in most countries. However, it is impossible to manage the factors that effect drying, including humidity, temperature, mass flow rate for drying, and heat entering the drier chamber. As a result, it causes an unfavorable drying speed or a longer drying time. The OSD approach has additional drawbacks, such as lowering product quality due to wind, trash, rain, insects, and animals [1, 2]. Fossil fuel combustion often produces hot air for industrial drying, and huge amounts of energy (about 13% in the agricultural industry) are used globally for this purpose [3]. The quality of the dried product is one of the key factors in the drying process. Therefore, the hot air used to dry the product should be in the range of 45 to 60$°\mathrm{C}$. The intensity of solar radiation decreases during sunset hours and the process of drying and dehumidifying the product continues in the dryer chamber, so dryers must dry the product continuously. As it was said, the quality of the dried product is important, so the temperature inside the drying chamber should not be too high, also at noon, when the solar radiation increases. The optimum option might be the thermal storage system (TES), according to the literature review. PCMs (phase change materials), can be used in the solar dryer system, the most important PCM can be mentioned paraffin wax. Numerous studies based on solar dryers of various technologies have been carried out, and the majority of them compared the results of a solar-assisted drying method with a conventional method. When compared to the conventional drying method, drying wet materials with solar dryers reduces drying time while improving dried sample quality [1, 2, 3]. Table 1 shows new studies on the technology of PCM-equipped solar dryers. In addition, the key results of each research are given separately. The use of heat during the drying process could result in significant changes in the product quality. Low temperatures are thus preferred, particularly for drying medical plants, fruits, and vegetables. Low temperature necessitates the use of a high-quality energy source, which significantly raises irreversibility [2]. Therefore, it is essential to increase drying energy efficiency and reduce the costs related to energy use in this sector. Increasing energy efficiency and reducing operating costs is more possible with the use of new technologies. For this reason, recent studies have focused on increasing energy efficiency and reducing the operating costs of drying systems with renewable energy sources. Energy cost evaluations for various drying methods are the primary focus of many studies today. Exergy economic analysis, another name for this method, ensures that the main causes of the system’s exergy loss are clearly identified. This makes it possible to pinpoint the system component that requires improvement and to put the right methods into action to boost the exergy efficiency.

Table 1.

A review of new technologies of solar dryers equipped with PCM.

2. Solar dryers

Solar dryers are divided into two categories: forced convection (FC) and natural convection (NC) based on the air circulation employed for drying [8, 9]. As a result, altering the drying method alters the characteristics of agricultural samples during the drying process. The hot air required in the drying process is supplied to the drying chamber through a solar air heater using an external device such as a fan. Forced circulation solar dryers are also known as active solar dryers. The airflow needed to dry the samples in natural circulation dryers (or inactive solar dryers) is caused by gravity or buoyant force. Figure 1 depicts the classification of solar dryers with natural and forced circulation into three categories: direct, indirect, and mixed-mode. Based on the ways of drying displayed in Figure 1, a list of the solar dryers available for drying agricultural commodities [10, 11, 12].

Figure 1.

Classification of solar dryers and drying techniques [10].

The direct, indirect, and mixed-mode categories of solar dryers are described in the preceding section [5, 10]. Dryer chambers are present in direct sun dryers. This chamber is comprised of an opaque cover and is insulated. Usually, the drying chamber is made of materials such as glass, plastic, and galvanized sheets, and in order for air to enter the drying chamber and the product to dry, holes are placed at the entrance and exit of the chamber [13, 14]. Natural sun-drying (A1), Natural rack or shade drying (A2), Staircase solar drying (A3), and Foldable solar drying (A4) are examples of direct solar drying (Figure 2), which can also be seen that the product is exposed to direct sunlight, so the quality of the dried products is low and Environmental parameters cannot be controlled [10, 15]. One of the advantages of direct solar dryers is that they are economical because they are easy to make and have a low cost. This dryer can protect the product from dust, wind, and rain. The main disadvantages of these dryers can be mentioned as the lack of control of environmental factors such as radiation intensity, speed, temperature entering the dryer chamber, and long drying time and reducing the quality of the dried product [16]. To overcome the disadvantages of direct solar dryers, the hot air needed to dry the product was provided by solar air heaters. Therefore, in indirect solar dryers, sunlight does not shine directly on the product inside the dryer chamber. A blower or suction fan can be added to indirect solar dryers to create a forced convection to dry the product [17]. Higher temperature values can be attained with regulated airflow rates using the solar collector unit [18]. The speed of air movement in the solar collector, which can be managed by a fan or blower, affects the effectiveness of an indirect solar drier. A fan’s principal function in a dryer unit is to maintain the correct airflow rate, which promotes even moisture removal from products [19]. Solar Dryers with Natural circulation (B1), with Natural circulation and Chimney phenomenon (B2), Integrated roof (B3), and greenhouse (B4) are examples of indirect solar dryers (Figure 2), whose common feature is the supply of hot air by a solar air heater, which at the end it is connected to the dryer chamber [10]. In mixed-mode solar dryers, due to the simultaneous use of the solar air heater and the drying chamber against direct sunlight, the drying efficiency raises and reduces the drying time of the product. Solar drying tunnel (C) and hybrid dryers belong to the mixed-mode category [10].

Figure 2.

Classification of types of solar dryers for drying [10].

3. Calculation of exergy in solar dryers

There are various components in solar dryers, such as solar collector, fan, and dryer chamber, so many factors may play a role in exergy due to the presence of these components in the dryer system. The optimization of each of these components is determined using exergy analysis and one of these parameters is the calculation of exergy loss. According to the results obtained from exergy analysis, suction fans in solar dryers have the highest exergy destruction and the lowest exergy efficiency [2, 20].

Important factors in the drying process can be mentioned sunlight, the mass flow rate of air, and humidity inside the drying chamber. Exergy loss decreases with the increase of air mass flow rate, so the effect of humidity on exergy changes is insignificant [21]. Solar dryers have rather poor exergy efficiency, according to an examination of energy and exergy are done on them [22]. The calculations do not consider the effects of kinetic and potential energy, and the exergy balance of a product can be calculated from Eq. (1):

$$E_i-E_o=E_{\mathit{\text{dest}}}$$E1

The ${\mathit{E}}_{\mathit{\text{dest}}}$, which indicates the destructive exergy for the drier throughout the drying phase of the slices, can be represented as (Eq. 2):

$$E_{\mathit{\text{dest}}}={\mathit{Ex}}_{\mathit{\text{in}}}+\mathrm{W}-{\mathit{Ex}}_{u,p}$$E2

${\mathit{Ex}}_{u,p}$ is the real exergy in Eq. (2) and may be determined using the pressure drop in the collector:

$${\mathit{Ex}}_{u,p}={\mathit{Ex}}_u-{\mathit{Ex}}_{\mathrm{w}}$$E3

Input (${\mathit{Ex}}_{\mathit{\text{in}}}$) and output exergy (${\mathit{Ex}}_u$) parameters can be calculated from Eqs. (4) and (5) [2]:

$${\mathit{Ex}}_{\mathit{\text{in}}}=\dot{\mathrm{m}}{\mathrm{c}}_{\mathrm{p}}\left[\left({\mathrm{T}}_{\mathrm{o}}-{\mathrm{T}}_{\mathrm{i}}\right)-{\mathrm{T}}_{\mathrm{a}}\left(\mathrm{Ln}\frac{{\mathrm{T}}_{\mathrm{o}}}{{\mathrm{T}}_{\mathrm{i}}}\right)\right]$$E4

$${\mathit{Ex}}_u=\dot{\mathrm{m}}\left[c_p\left(T_o-T_i\right)-T_a\left(c_v\mathit{Ln}\left(\frac{T_o}{T_i}\right)-\mathit{RLn}\left(\frac{{\rho }_{\mathit{\text{out}}}}{{\rho }_{\mathit{\text{in}}}}\right)\right)\right]$$E5

The dryer’s pump and fan are tied to ${\mathit{Ex}}_{\mathrm{w}}$, which represents the system’s additional source of energy.

${\mathit{Ex}}_{\mathrm{w}}$ For the pump was estimated using a wattmeter, while ${\mathit{Ex}}_{\mathrm{w}}$ for the fan was computed using equation [2]:

$${\mathit{Ex}}_{\mathrm{w}}=\frac{{\mathrm{T}}_{\mathrm{a}}}{{\mathrm{T}}_{\mathrm{i}}}{W}_{\mathrm{fan}},W_{\mathit{fan}}=\frac{\dot{m}\times ∆P}{\left(\rho \times {\eta }_{\mathit{fan}}\right)}$$E6

η Fan indicated the fan efficiency value, which for the current system was 0.91.

Eq. (7) can be used to determine the exergy efficiency of collectors while taking the sun’s temperature (4350 K) and the inlet and output fluid temperatures into account [2, 23]:

$${\eta }^{\prime }=\frac{\dot{m}{c}_p\left[\left(T_o-T_i\right)-T_a\left(\mathit{ln}\frac{T_a}{T_i}\right)\right]}{A_c{I}_o\left[1-\frac{T_a}{T_{\mathit{sun}}}\right]}$$E7

Additionally, the results of Eqs. (8) and (9) [24], which were used to calculate the exergy efficiency for the drying chamber and the drying process, are as follows:

$${\eta }_{\mathrm{dry},\mathrm{cab}}^{\prime }=\frac{\left[1-\frac{T_o}{T_{\mathit{\text{in}}}}\right]{\dot{Q}}_{o,\mathit{\text{dryer}}}}{{\mathit{Ex}}_{\text{in}}}$$E8

$${\eta }_{\mathrm{dry}}^{\prime }=\frac{\dot{\mathrm{m}}{\mathrm{c}}_{\mathrm{p}}\left[\left({\mathrm{T}}_{\mathrm{o}}-{\mathrm{T}}_{\mathrm{a}}\right)-{\mathrm{T}}_{\mathrm{a}}\left(\mathrm{Ln}\frac{{\mathrm{T}}_{\mathrm{o}}}{{\mathrm{T}}_{\mathrm{a}}}\right)\right]}{\dot{\mathrm{m}}{\mathrm{c}}_{\mathrm{p}}\left[\left({\mathrm{T}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{a}}\right)-{\mathrm{T}}_{\mathrm{a}}\left(\mathrm{Ln}\frac{{\mathrm{T}}_{\mathrm{i}}}{{\mathrm{T}}_0}\right)\right]}$$E9

4. A review of the results of articles on exergy solar dryers

In a study, a solar dryer system fitted with PCM underwent an experimental examination and an exergy analysis. Figure 3a shows exergy efficiency and exergy loss on the test days. Due to the use of thermal energy storage used in this paraffin wax test, exergy loss occurs less during daylight hours. The results show that the lowest exergy loss with 0.18117 kW and the highest exergy efficiency with 69.59% is related to the first day of the experiment. On the second and third days, exergy loss increases due to the amount of drying of the product and the change in environmental conditions. Economic exergy analysis is shown in Figure 3b. It can be concluded that the highest cost of exergy destruction is related to the fans in solar dryers and they have a minimum exergy efficiency of 55.28%. Therefore, the fans in the dryers should be optimized [20].

Figure 3.

(a) Values of exergy efficiency and exergy loss during the test days, (b) Exergeoeconomic analysis for the solar dryer system in this research [20].

In other study, the exergy efficiency for the drying process Jerusalem Artichoke slices with the dryer integrated with PCM varies between 35.3–59.7% (Figure 4b) and, for the system without PCM ranged between 17.1 to 42.9% (Figure 4b). This means that PCM usage improved the exergy efficiency (at least 28.62%). In both cases with and without PCM, exergy loss and exergy destruction increase with increasing air mass flow rate (Figure 4a) [2]. The specific energy consumption (SEC) of 2.62 kWh/kg was discovered in another study. Additionally, the average energy efficiency of solar drying was 30%, with a range of 1 to 93% (Figure 5). Improvement potential values were shown to be between 0.3 and 630 W, with an average of 247 W (Figure 5) [25].

Figure 4.

(a) Exergy loss (EL), Exergy input (ETR), and Exergy destruction (DE) parameters for solar dryers equipped with PCM for different mass flow rates, (b) Exergy efficiency values for the cases with and without PCM for the drying chamber and the drying process [2].

Figure 5.

Changes in recovery potential and exergy efficiency during the test period [25].

For both configurations, the exergy inflow, outflow, and losses were calculated for the collector and drying chamber. In forced and natural convection modes, the exergy outflow of the SAC was between 1.04 and 46.85 W and 1.13 and 50.94 W, respectively (Figure 6a). Under forced and natural convection, the exergy loss for the drying chamber ranged from 0.062 to 21.99 W and 0.394 to 24.99 W, respectively (Figure 6b). Therefore Average exergy efficiencies for SAC and drying chamber in ISD with forced convection were 2.03 and 59.32%, while these values were 2.44 and 55.45% in ISD with natural convection [26]. In another study, energy and exergy evaluations were used to determine how well each component of the dewatering system performed. The calculated mean values of the SAC’s energy and exergy efficiencies were 9.8 (26.10%) and 0.14 (0.81%), respectively [27].

Figure 6.

(a) Exergy output for forced and natural convection from solar air heater (SAH), (b) exergy losses from solar air heater (SAH) [26].

In another study, an experimental study was conducted on an indirect solar dryer (ISD). The drying process has been carried out at different flow rates. The findings indicate that when air mass flow rate increases, exergy efficiency rises, and exergy losses fall. As shown in Figure 7, the energy efficiency rose from 28.74 to 40.67% with the increase in mass flow rate from 0.0141 to 0.0872 kg/s [28].

Figure 7.

Exergy losses and exergy efficiency in different mass flow rates during the test period.

5. Conclusion

In this article, a review of the mechanism of solar dryers with drying technologies with PCM, exergy calculation formulas, exergy losses, and exergy destruction was investigated. In addition, new articles about the results of exergy analysis and parameters such as exergy efficiency, losses, and exergy destruction for new solar dryers were reviewed, the important results of which are:

• Indirect solar dryers with forced convection have more exergy losses due to the presence of a suction fan and have higher exergy efficiency compared to natural convection.

• The use of PCM in solar dryers significantly improves exergy efficiency. Because the most important problem of solar dryers is the continuous drying of the product in the afternoon until the night, to solve this problem, a thermal storage system is used. The most commonly used PCM is paraffin wax, which is connected to copper pipes inside the storage tank. They are placed in the solar collector or inside copper or aluminum boxes and placed inside the drying chamber or inside the solar air heater.

• With an increase in air mass flow rate, exergy efficiency rises and exergy loss falls [2, 20].

• According to the findings of prior studies, the fans used in solar dryers have the largest exergy destruction and the lowest exergy efficiency.

• Exergy efficiency can be increased by optimizing the suction fans or the air recycling system can be used in solar dryers [2, 29].

• Humidity is one of the important factors in the drying process. The drying process is carried out slowly because the final moisture content of the product must be reached and the product dried, so the volume of the incoming air is small compared to changes in humidity, so the humidity changes in exergy are insignificant.

Nomenclature

Exu,p	exergy outlet considering pressure drop (W)
Exdest	exergy destruction (W)
Exin	exergy inlet (W)
Exu	exergy outlet without considering pressure drop (W)
Exw	exergy for work (W)
ṁ	Mass flow rate (kg/s)
Ta	Air temperature (°C)
To	Temperature outlet (°C)
Ti	Temperature inlet (°C)
Ac	Collector area (m2)
Io	Solar radiation normal to the collector (W/m2)
Q̇o,dryer	Energy output from the chamber dryer (J)
ρ	Density (kg/m3)

Abbreviations

TES	Thermal energy storage
PCM	Phase change material
SEC	Specific energy consumption
ISD	Indirect solar dryer
SAC	Solar air collector