New Technologies in Exergy

#### **Chapter 1**

## Exergy of Solar Dryer

*Mohammad Saleh Barghi Jahromi*

#### **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.

**Keywords:** solar dryer, exergy efficiency, exergy loss, renewable energy, drying techniques

#### **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°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 [1–3]. **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,



[6] Parabolic Trough Solar Collector (PTSC) - paraffin wax is located in the copper coil in the expansion tank.

Apple The fluids used in this test include PCM, engine oil, glycerin, and water, which are located in the copper pipes located in the expansion source. The highest temperature is 83.8, 88.2, 85.6, and 79.2 degrees Celsius for Nano fluid, oil, glycerin, and water, respectively. The efficiency of the collector is in the range of 71.9 to 73.4%.

[7] Solar air heater (SAH) - paraffin wax is located under the absorber plate.

banana Banana slices were to be dried for 18 hours, of which 13 hours were spent during the day and 5 hours were spent draining PCM material. - Average drying efficiency was found to be 2.98%, and collector efficiency to be 66.32%.

#### **Table 1.**

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

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.

#### **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

**Figure 1.** *Classification of solar dryers and drying techniques [10].*

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 [10–12].

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

**Figure 2.** *Classification of types of solar dryers for drying [10].*

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].

#### **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):

$$E\_i - E\_o = E\_{dest} \tag{1}$$

The *Edest*, which indicates the destructive exergy for the drier throughout the drying phase of the slices, can be represented as (Eq. 2):

$$E\_{\text{det}} = E\mathbf{x}\_{\text{in}} + \mathbf{W} - E\mathbf{x}\_{\text{tr}} \tag{2}$$

*Exu*,*<sup>p</sup>* is the real exergy in Eq. (2) and may be determined using the pressure drop in the collector:

$$E\mathbf{x}\_{\mathbf{u},\mathbf{p}} = E\mathbf{x}\_{\mathbf{u}} - E\mathbf{x}\_{\mathbf{w}} \tag{3}$$

Input (*Exin*) and output exergy (*Exu*) parameters can be calculated from Eqs. (4) and (5) [2]:

$$E\mathbf{x}\_{\dot{m}} = \dot{\mathbf{m}}\mathbf{c}\_{\mathbf{p}} \left[ (\mathbf{T}\_{\mathbf{o}} - \mathbf{T}\_{\dot{i}}) - \mathbf{T}\_{\mathbf{a}} \left( \mathbf{L}\mathbf{n}\frac{\mathbf{T}\_{\mathbf{o}}}{\mathbf{T}\_{\dot{i}}} \right) \right] \tag{4}$$

$$E\mathbf{x}\_{u} = \dot{\mathbf{m}} \left[ c\_{p} (T\_{o} - T\_{i}) - T\_{a} \left( c\_{v} L n \left( \frac{T\_{o}}{T\_{i}} \right) - R L n \left( \frac{\rho\_{out}}{\rho\_{in}} \right) \right) \right] \tag{5}$$

The dryer's pump and fan are tied to *Ex*w, which represents the system's additional source of energy.

*Ex*<sup>w</sup> For the pump was estimated using a wattmeter, while *Ex*<sup>w</sup> for the fan was computed using equation [2]:

$$E\mathbf{x}\_{\mathbf{w}} = \frac{\mathbf{T}\_{\mathbf{a}}}{\mathbf{T}\_{\mathbf{i}}} \mathbf{W}\_{\mathbf{fan}}, \mathbf{W}\_{fan} = \frac{\dot{m} \times \Delta P}{\left(\rho \times \eta\_{fan}\right)} \tag{6}$$

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]:

$$\eta' = \frac{\dot{m}c\_p \left[ (T\_o - T\_i) - T\_a \left( \ln \frac{T\_a}{T\_i} \right) \right]}{A\_c I\_o \left[ 1 - \frac{T\_a}{T\_{\text{mu}}} \right]} \tag{7}$$

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:

$$\eta'\_{\rm dry,cab} = \frac{\left[\mathbf{1} - \frac{T\_s}{T\_{\rm in}}\right] \dot{Q}\_{o,dryer}}{E \mathbf{x}\_{\rm in}} \tag{8}$$

$$\eta\_{\rm dry}^{\prime} = \frac{\dot{\rm inc}\_{\rm p} \left[ (\mathbf{T}\_{\rm o} - \mathbf{T}\_{\rm a}) - \mathbf{T}\_{\rm a} \left( \mathbf{Ln} \frac{\mathbf{T}\_{\rm a}}{\mathbf{T}\_{\rm a}} \right) \right]}{\dot{\rm inc}\_{\rm p} \left[ (\mathbf{T}\_{\rm i} - \mathbf{T}\_{\rm a}) - \mathbf{T}\_{\rm a} \left( \mathbf{Ln} \frac{\mathbf{T}\_{\rm i}}{\mathbf{T}\_{\rm o}} \right) \right]} \tag{9}$$

#### **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].

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

#### *Exergy of Solar Dryer DOI: http://dx.doi.org/10.5772/intechopen.109082*


**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].*

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].

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

#### **Figure 6.**

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

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].

#### **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

**Figure 7.** *Exergy losses and exergy efficiency in different mass flow rates during the test period.*

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:


## **Nomenclature**


## **Abbreviations**


## **Author details**

Mohammad Saleh Barghi Jahromi Faculty of Mechanical Engineering, Department of Mechanical Engineering, Yazd University, Iran

\*Address all correspondence to: msc.barghi@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **References**

[1] Mokhtarian M, Tavakolipour H, Ashtari AK. Effects of solar drying along with air recycling system on physicochemical and sensory properties of dehydrated pistachio nuts. LWT. 2017;**75**:202-209

[2] Jahromi MSB, Iranmanesh M, Akhijahani HS. Thermo-economic analysis of solar drying of Jerusalem artichoke (Helianthus tuberosus L.) integrated with evacuated tube solar collector and phase change material. Journal of Energy Storage. 2022;**52**: 104688

[3] Barghi Jahromi MS, Iranmanesh M. Experimental investigation on the use of PCM in a pistachio solar dryer by the evacuated heat pipe solar collector. Journal of Pistachio Science and Technology. 2019;**3**(6):73-87

[4] Bhardwaj AK, Chauhan R, Kumar R, Sethi M, Rana A. Experimental investigation of an indirect solar dryer integrated with phase change material for drying valeriana jatamansi (medicinal herb). Case Studies in Thermal Engineering. 2017;**10**:302-314

[5] Iranmanesh M, Akhijahani HS, Jahromi MSB. CFD modeling and evaluation the performance of a solar cabinet dryer equipped with evacuated tube solar collector and thermal storage system. Renewable Energy. 2020;**145**: 1192-1213

[6] El Khadraoui A, Bouadila S, Kooli S, Farhat A, Guizani A. Thermal behavior of indirect solar dryer: Nocturnal usage of solar air collector with PCM. Journal of Cleaner Production. 2017;**148**:37-48

[7] Singh D, Mall P, . Experimental investigation of thermal performance of indirect mode solar dryer with phase

change material for banana slices. Energy Sources, Part A Recover. Utilization, and Environmental Effects. 2020:1-18. DOI: 10.1080/ 15567036.2020.1810825

[8] Mohana Y, Mohanapriya R, Anukiruthika T, Yoha KS, Moses JA, Anandharamakrishnan C. Solar dryers for food applications: Concepts, designs, and recent advances. Solar Energy. 2020; **208**:321-344

[9] Lingayat AB, Chandramohan VP, Raju VRK, Meda V. A review on indirect type solar dryers for agricultural crops– Dryer setup, its performance, energy storage and important highlights. Applied Energy. 2020;**258**:114005

[10] Jahromi MSB, Kalantar V, Akhijahani HS, Kargarsharifabad H. Recent progress on solar cabinet dryers for agricultural products equipped with energy storage using phase change materials. Journal of Energy Storage. 2022;**51**:104434

[11] Ekechukwu OV, Norton B. Review of solar-energy drying systems II: An overview of solar drying technology. Energy Conversion and Management. 1999;**40**(6):615-655

[12] Basunia MA, Abe T. Thin-layer solar drying characteristics of rough rice under natural convection. Journal of Food Engineering. 2001;**47**(4):295-301

[13] Ghazanfari A, Tabil L Jr, Sokhansanj S. Evaluating a solar dryer for in-shell drying of split pistachio nuts. Drying Technology. 2003;**21**(7): 1357-1368

[14] Seveda MS, Jhajharia D. Design and performance evaluation of solar dryer

for drying of large cardamom (Amomum subulatum). Journal of Renewable and Sustainable Energy. 2012;**4**(6):063129

[15] Sharma A, Chen CR, Lan NV. Solarenergy drying systems: A review. Renewable and Sustainable Energy Reviews. 2009;**13**(6–7):1185-1210

[16] Hii CL, Law CL. Solar drying of major commodity products. In: Solar Drying: Fundamentals, Applications and Innovations. 2012. pp. 73-94

[17] Green MG, Schwarz D. Solar Drying Technology for Food Preservation. Germany: GTZ Publication Eschborn; 2001

[18] Fadhel MI, Abdo RA, Yousif BF, Zaharim A, Sopian K. Thin-layer drying characteristics of banana slices in a force convection indirect solar drying. In Proceedings of the 6th IASME/WSEAS International Conference on Energy and Environment (EE 2011). World Scientific and Engineering Academy and Society Press. February 2011. pp. 310-315

[19] Ghatrehsamani SH, Zomorodian A. Impacts of drying air temperature, bed depth and air flow rate on walnut drying rate in an indirect solar dryer. International Journal of Agriculture Sciences. 2012;**4**(6):253

[20] Atalay H, Cankurtaran E. Energy, exergy, exergoeconomic and exergoenvironmental analyses of a large scale solar dryer with PCM energy storage medium. Energy. 2021;**216**:119221

[21] Akbulut A, Durmuş A. Energy and exergy analyses of thin layer drying of mulberry in a forced solar dryer. Energy. 2010;**35**(4):1754-1763

[22] Sami S, Etesami N, Rahimi A. Energy and exergy analysis of an indirect solar

cabinet dryer based on mathematical modeling results. Energy. 2011;**36**(5): 2847-2855

[23] Petela R. Exergy of heat radiation. Transactions of the ASME: Journal of Heat Transfer. 1964;**2**:187-192

[24] Tagnamas Z, Kouhila M, Bahammou Y, Lamsyehe H, Moussaoui H, Idlimam A, et al. Drying kinetics and energy analysis of carob seeds (*Ceratonia siliqua* L.) convective solar drying. Journal of Thermal Analysis and Calorimetry. 2022;**147**(3):2281-2291

[25] Fudholi A, Sopian K, Othman MY, Ruslan MH. Energy and exergy analyses of solar drying system of red seaweed. Energy and Buildings. 2014;**68**:121-129

[26] Mugi VR, Chandramohan VP. Energy and exergy analysis of forced and natural convection indirect solar dryers: Estimation of exergy inflow, outflow, losses, exergy efficiencies and sustainability indicators from drying experiments. Journal of Cleaner Production. 2021;**282**:124421

[27] Bhardwaj AK, Kumar R, Kumar S, Goel B, Chauhan R. Energy and exergy analyses of drying medicinal herb in a novel forced convection solar dryer integrated with SHSM and PCM. Sustainable Energy Technologies and Assessments. 2021;**45**:101119

[28] Vijayan S, Arjunan TV, Kumar A. Exergo-environmental analysis of an indirect forced convection solar dryer for drying bitter gourd slices. Renewable Energy. 2020;**146**:2210-2223

[29] Atalay H. Exergoeconomic and environmental impact evaluation of wind energy assisted hybrid solar dryer and conventional solar dryer. Renewable Energy. 2022;**200**:1416-1425

#### **Chapter 2**
