Introduction

Since ancient times, Krishna tulsi has been considered a prominent medicinal and aromatic herb belonging to the Lamiaceae family. Krishna tulsi leaves are used for throat infections, skin diseases, respiratory health improvement, and to treat fever. It is a tropical plant native to India and widely cultivated across Southeast Asia. Tulsi is grown for religious and traditional medicinal purposes and for its essential oil. It is popular as a herbal tea, particularly in Ayurvedic medicine. Dried Krishna tulsi leaves are very effective in various health conditions like cold, cough, tight chest, etc. (Bhadra 2020) (Jamshidi and Cohen 2017).

Nowadays, Krishna tulsi leaves are extensively cultivated in countries including France, the United States, Greece, Morocco, Israel, Indonesia, and Hungary. Dried leaves are commonly used in food processing industries. The market for dried leaves is determined based on the quality of the product based on taste, flavor, moisture content, and appearance. From the dried ones, several products like tulsi powder, tea, and capsules are made after processing (Khatun et al. 2014). Regular consumption of taking tulsi powder improves the body’s immunity, relieves stress, promotes a healthy metabolism, and reduces blood sugar levels in patients with type-2 diabetes (Jamshidi and Cohen 2017). Handling leaves after harvest determines the end quality and aromatic properties. Color, aroma, and texture are the three key factors in grading product quality on the market. Krishna tulsi leaves are scarce during the off-season. Preservation of these medicinal herbs is required for year-round use. Hence, a proper drying method is required for Krishna tulsi leaves for better storage with less spoilage and higher shelf life (Bhaskara Rao and Murugan 2021) (Babu et al. 2018).

The concept of drying is a process of removing moisture by combining heat and mass transfer. Generally, any food product can be dried by open-sun drying (OSD), which spreads the food product in an open area and is directed toward the sun. Even though it is a simple and economical drying method, it has the following drawbacks; (i) improper drying, (ii) color degradation, (iii) collecting dust and dirt, etc., on the resulting product (Dhanore et al. 2018) (Tomar et al. 2017). Due to hostile weather conditions, open-sun drying always has drawbacks. On the other hand, thermal and electrical dryers consume huge electrical energy and require higher initial investments (Kondareddy et al. 2022). Electrical energy consumption indirectly increases CO2 emissions, which increase global warming potential (GWP). Due to this, drying food products using cheaper and cleaner energy is a focus of research by various researchers. The utilization of clean and alternative renewable sources has become realistic in the last two decades. In this context, solar energy is abundantly available compared to other renewable energy sources. The application of solar energy is rapidly increasing in different applications, including heating, cooling, power generation, drying, etc., due to its economic, eco-friendly, and availability (Vijayan et al. 2020). A key part on the performance of an efficient solar drying system is the solar thermal collector. The hot air from the collector is the main factor in how well the dryer reduces moisture from the drying product. In this scenario, an ETSC delivers relatively higher temperatures than a flat plate collector. It achieves higher temperatures even during low sun-shining days than flat plate collectors. ETSC’s working temperature range is 40–100 °C, and it is sufficient for drying process. ETSC is flexible if a tube breaks, it can be easily replaced. The maintenance cost is low, and the performance of these collectors is good in cold conditions (Olfian et al. 2020).

Danso-Boateng (2013) performed drying experiments on basil leaves using five different methods, namely, microwave drying, oven drying, hot air drying, sun drying, and ambient air drying to assess the nutritional content. Among all the methods, the microwave drying method took less drying time with the most inhibition of protein and carbohydrate content in the dried leaves. Daghigh and Shafieian (2016) performed energy and exergy analyses on an evacuated heat pipe collector for dual-purpose usage by using a heat exchanger for water heating and drying applications. The system’s exergetic performance increased with time and increased to a maximum of about 10.6% at the end of the day. Singh et al. (2022) investigated an ETSC dryer used for drying turmeric, and the thermal efficiency varies from 18.5 to 22.5%. The required time for drying in the greenhouse dryer is reduced by about 55% in comparison with OSD, and the quality of the dried turmeric is good in terms of curcumin, volatile content, and polishing loss. The payback period of energy and costs is reported as 2.95 and 1.10 years, respectively. CO2 mitigation is given as 209.21 tons.

Malakar et al. (2021) experimented with drying garlic cloves in an ETSC solar dryer with different air flow rates of 1, 2, and 3 m/s. The average exergy efficiency is found to be 56.6%, at a flow rate of 2 m/s, and the highest exergy loss is found at 1 m/s as 4.74 W. The dryer payback period is 1.3 years. In another study, the same authors (Malakar et al. 2022) performed the drying process on the beetroot slices in an ETSC; the final moisture content in the beetroot slices is 9.25%. The highest color change is found in OSD samples, whereas antioxidant activity and phenolic compounds are higher in ETSC samples. Singh and Gaur developed ETSC greenhouse dryer and conducted experiments on drying ginger and tomatoes. The values of the convective heat transfer are ranging between 2.9 and 3.33. Various authors (Sadhishkumar and Balusamy 2014) (Sadeghi et al. 2020) (Das and Akpinar 2020) (Shrivastava et al. 2022) evaluated the value of convective heat transfer for different food products by using different solar dryers and found its significance while designing efficient dryers.

Energy and exergy analyses of an indirect solar dryer with natural and forced convection modes for drying Okra are performed by Mugi and Chandramohan (2021). The dryer’s efficiency is higher by about 5% in the forced convection mode. The exergy analysis indicates that drying chamber exergy losses are high in the natural convection mode. The average collector and drying efficiencies are 2.03, 59.32%, 2.44, and 55.45%, in the forced and natural convection modes of drying, respectively. Lakshmi et al. (2019) dried stevia leaves by using a solar dryer in a mixed mode, and the desired moisture content is obtained in 5.5 and 14.5 h in the mixed mode and OSD, respectively. The average dryer exergy efficiency is 59.1%. Antioxidants and flavonoids are higher in the mixed mode, and the payback period is reported as 0.65 years.

The suitable design and optimization of the solar dryer result in maximum moisture reduction of the product with lesser energy utilization. The amount of quality and quantity of energy utilized during the drying process determine the performance of the system. Exergy analysis of a solar drying system indicates the quality of the energy utilized in the drying process, which is also determined by energy analysis. The first law of thermodynamics is used for the energy analysis of drying. It shows how energy is conserved by the various parts of the drying system.

Numerous works related to the drying of food products and medicinal herbs are reported in the literature (Mghazli et al. 2017) (Kabeel et al. 2022) (Rani and Tripathy 2021). Different research studies are carried out on the thin-layer drying of fruits, vegetables, and herbs (Ouaabou et al. 2020) (Vijayan et al. 2016) (Lingayat et al. 2021) (Bahammou et al. 2019b). Different authors studied the thermal performance, energy analysis, and exergy analysis (Tiwari and Tiwari 2017) (Bhardwaj et al. 2019) (Bhaskara Rao and Murugan 2021), environmental analyses (Moosavian et al. 2021) (Srivastava et al. 2021) (Kesavan et al. 2019), and economic analysis (Singh et al. 2006) (Jain and Tewari 2015) (Lakshmi et al. 2019) (Devan et al. 2020) on the solar drying systems. But, a number of energy, exergy, environmental, and economic research studies related to the ETSC-supported dryer are not reported in the literature. And also, the performance evaluation of an ETSC used for drying Krishna tulsi has not yet been done. Based on this research gap, experimental studies are carried out on drying Krishna tulsi using an ETSC supported dryer and then performed the 4E analysis on the newly fabricated dryer.

Novelty and objectives of the research study

The novelty and objectives of the present study aim to evaluate the following: (i) thus far, no studies have been found on drying Krishna tulsi using a solar dryer. Therefore, experiments were carried out on drying Krishna tulsi, and the thermal performance of the ETSC dryer was assessed. (ii) The estimation of the ETSC dryer’s energy inflow, outflow, and sustainability indicators was not reported in the literature. As a result, the study is aimed at evaluating the energy and exergy analysis of the overall drying process in order to determine energy utilization and at determining where thermodynamic improvements can be made by using sustainability indices, and (iii) an environmental and economic analysis of the proposed dryer is performed to assess the environmental impact and cost-related parameters of the novel ETSC dryer.

Materials and methods

Experimental setup

An in-house fabricated U-Tube ETSC combined with an indirect drying chamber was positioned on the Mechanical Engineering Block, NIT Rourkela, India. Figure 1 depicts a schematic view of the experimental test setup. The drying experiments were conducted in January 2022. In each batch, 4 kg of Krishna tulsi leaves were dried. Three batches of Krishna tulsi leaves drying studies were performed. The photo of the fabricated ETSC dryer is displayed in Fig. 2.

Fig. 1
figure 1

Schematic view of the experimental test setup

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Fig. 2
figure 2

Photograph of the experimental setup

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The experimental setup comprises (a) an ETSC collector with U-Tube, (b) an indirect drying chamber, and (iii) a centrifugal blower. The ETSC collector is an active mode forced convection type. The air is forced into the collector by an electric blower (0.28 HP, 20 V, 50 Hz, 2800 RPM). An air blower blows atmospheric air into the inlet manifold of the ETSC collector. Air enters every tube of the collector and is heated up by solar radiation that falls on the glass tubes. Hot air leaves the collector’s outlet manifold. The inlet and outlet manifolds are sealed with 25 mm thickness of glass wool to reduce heat losses. Thermocouples are placed on both manifolds to measure temperatures at different locations. The collector is directed south with a 19.27° inclination angle.

An indirect drying chamber is made up of galvanized iron sheets, and glass wool is used to insulate it on all sides. Four trays are placed in the drying chamber with equal spacing of 150 mm between them. The dimensions of each tray are 500 × 1000 mm. The product to be dried is placed on drying chamber trays. Four RTD-type thermocouples are installed near each tray in the drying chamber to measure the temperature, and all thermocouples are connected to a temperature data logger. The heated air will be transferred to the bottom of the drying cabinet from the ETSC. Hot air passes upward through each tray in the chamber. Then, the moist air leaves through a chimney at the top of the drying chamber. Forced convection solar drying has a higher drying rate than natural convection dryers. The flow rate of the drying air in the chamber is maintained at 0.03 kg/s, and this is controlled by regulating the ball valve placed between the blower and collector inlet manifold. The ETSC and drying cabinet specifications are listed in Table 1.

Table 1 ETSC and drying cabinet specifications
Full size table

Sample preparation and experimentation

Krishna tulsi leaves are available from March to June in spring. Fresh Krishna tulsi leaves were bought from the local market and rinsed with water. After cleaning the leaves, they are spread over a place to remove water particles accumulated on the surface of the leaves. The leaves are kept in a room for one day before drying experiment. The initial moisture content of Krishna tulsi leaves is calculated by the hot air oven drying method (Kesavan et al. 2019). Fresh Krishna tulsi leaves are spread over the drying trays, and a portion of leaves is also dispersed in the open sun for drying, as shown in Fig. 3. The moisture loss in the leaves is calculated using a digital weighing scale for every hour. The trays are interchanged frequently during the drying process for uniform drying. Experiments are carried out until there is no change in the drying product weight in the ETSC and OSD. After drying, leaves are packed into propylene bags to prevent moisture absorption. The ambient and drying chamber relative humidity is recorded using a digital hygrometer. The thermocouples placed at the collector and different locations in the drying chamber are connected to a temperature data logger for continuous temperature measurement. A pyranometer is placed near the collector to measure solar radiation. The experiments are conducted at Rourkela (22.26° N, 84.85° E), India.

Fig. 3
figure 3

Tulsi leaves drying in the (a) drying chamber and (b) open sun

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Uncertainty of the experimentation

Uncertainty analysis of the entire experimental procedure is necessary to estimate errors in different measured quantities. These errors may be due to human errors in noting the data, data approximation, less accuracy in measuring devices, etc. In this experimental study, uncertainty is calculated for the temperatures and relative humidity of air, sample weight, solar radiation, and velocities of air (Dhalsamant et al. 2018). Table 2 gives details of the different instruments used in the current experiment.

Table 2 Experiment uncertainty details
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The uncertainty of the dependent (WR) and independent (w1, w2, …wn) variables is computed from the equation below (Goud et al. 2019) (Kesavan et al. 2019).

$${W}_{R}={\left[{\left(\frac{\delta R}{{\delta X}_{1}}{W}_{1}\right)}^{2}+{\left({\frac{\delta R}{{\delta X}_{2}}}W_{2}\right)}^{2}+{\left({\frac{\delta R}{{\delta X}_{n}}}W_{n}\right)}^{2}\right]}^\frac{1}{2}$$
(1)

where WR is the uncertainty of the estimated value and w1, w2, …wn are the errors in the independent variables. Table 3 shows the uncertainty calculation of all measuring devices.

Table 3 Uncertainty values of the measuring devices used in this study
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Performance evaluation

Moisture removal

The amount of moisture level present in Krishna tulsi leaves is measured by using Eq. (2) (Gilago et al. 2023)

$$MC\left(db\right)=\frac{{M}_{i}-{M}_{f}}{{M}_{f}}$$
(2)

Moisture ratio (MR)

The moisture ratio of Krishna tulsi leaves is estimated by Eq. (3) (Saxena and Gaur 2020)

$$MR=\frac{{M}_{t}-{M}_{e}}{{M}_{o}-{M}_{e}}$$
(3)
$$MR=\frac{{M}_{t}}{{M}_{o}}$$
(4)

where Mt and M0 are the moisture content at time t, and Eq. (4) can be used because of the continuous change in moisture content of the drying product.

Drying rate (DR)

The food product drying rate is determined by Eq. (5) (Tiwari and Tiwari 2017)

$$DR=\frac{\Delta M}{\Delta t}$$
(5)

where ΔM and Δt are mass loss in the product and time interval, respectively.

Energy analysis

The energy analysis of a drying system can be expressed by mass and energy conservation equations (Hancioglu et al. 2010) (Dincer 2011).

The equation for mass conservation of air is commonly expressed as

$$\sum {m}_{ai}=\sum {m}_{ao}$$
(6)

The equation for mass conservation is expressed as

$$Q-W=\sum {m}_{o}\left({h}_{o}+\frac{{v}_{o}^{2}}{2}+{Z}_{o}g\right)-\sum {m}_{i}\left({h}_{i}+\frac{{v}_{i}^{2}}{2}+{Z}_{i}g\right)$$
(7)

where Q and W are the rate of heat and work transfers respectively, mi and mo are mass flow rate of air at inlet and outlet, hi and ho are the enthalpies of air at inlet and outlet, vi and vo are the velocities of the air at the inlet and outlet, Zi and Zo are the height of the collector and drying chamber at inlet and outlet from the reference, and g is the gravitational energy.

Energy analysis of the collector

The useful energy gained by the solar collector is given in Eq. (8)

$${Q}_{c}={m}_{a}{C}_{p}\left({T}_{co}-{T}_{ci}\right)$$
(8)

The efficiency of ETSC collector can be determined by the Eq. (9).

$${\eta }_{c}=\frac{{m}_{a}{C}_{p}\left({T}_{co}-{T}_{ci}\right)}{{IA}_{c}}$$
(9)

Energy analysis of the drying chamber

The amount of heat utilized in the drying chamber (using a psychrometric chart) by Eq. (10)

$${Q}_{dc}={m}_{a,dc}{C}_{p}\left({h}_{dci,T}-{h}_{dco,T}\right)$$
(10)

where hdci and hdco are the enthalpies of the drying air at drying chamber inlet and outlet.

The efficiency of the drying cabinet is expressed by Eq. (11)

$${\eta }_{dc}=\frac{{Q}_{dc}}{IA}=\frac{{m}_{a,dc}{C}_{p}\left({h}_{dci,T}-{h}_{dco,T}\right)}{IA}$$
(11)

Exergy analysis

Exergy analysis of the drying system is carried out, where thermodynamic improvements can be made for efficiency by using thermodynamics second law. The values of exergy inflow, outflow, loss, and efficiency are calculated for the drying process. Exergetic efficiency is the proportion of the amount of exergy utilized to reduce the moisture of a product to the exergy given to the product during the drying process (Bahammou et al. 2019a) (Rani and Tripathy 2021).

The following assumptions are taken while performing the exergy analysis.

  1. (i)

    The flow is considered steady

  2. (ii)

    Potential and kinetic energy effects are neglected

  3. (iii)

    The velocity difference between the inlet and outlet is minimal

Exergy analysis of the collector

The exergy of the solar radiation falls on the ETSC by Eq. (12) (Ge et al. 2014)

$${Ex}_{rad,c}={A}_{c}I\left[{1-\frac{4}{3}\left(\frac{{T}_{a}}{{T}_{s}}\right)+\frac{1}{3}\left(\frac{{T}_{a}}{{T}_{s}}\right)}^{4}\right]$$
(12)

The exergy inflow to the collector is expressed by Eq. (13)

$${Ex}_{in,c}={m}_{a}{c}_{pa}\left[\left({T}_{ci}-{T}_{a}\right)-{T}_{a} \ In\left(\frac{{T}_{ci}}{{T}_{A}}\right)\right]$$
(13)

The exergy outflow of the collector is written by Eq. (14)

$${Ex}_{out,c}={m}_{a}{c}_{pa}\left[\left({T}_{co}-{T}_{a}\right)-{T}_{a} \ In\left(\frac{{T}_{co}}{{T}_{a}}\right)\right]$$
(14)

The exergy efficiency of the collector is expressed by Eq. (15)

$${\eta }_{Ex,c}=\frac{{Ex}_{out,c}}{{Ex}_{in,c}}$$
(15)

Exergy analysis of the drying chamber

The expression for the inflow of exergy to the drying chamber can be written by Eq. (16); (Sansaniwal et al. 2018) (Rabha and Muthukumar 2017)

$${Ex}_{in,dc}={m}_{a}{c}_{pa}\left[\left({T}_{dci}-{T}_{a}\right)-{T}_{a} \ In\left(\frac{{T}_{dci}}{Ta}\right)\right]$$
(16)

The outflow of exergy from a drying chamber can be written by Eq. (17)

$${Ex}_{out,dc}={m}_{a}{c}_{pa}\left[\left({T}_{dco}-{T}_{a}\right)-{T}_{a} \ In\left(\frac{{T}_{dco}}{{T}_{a}}\right)\right]$$
(17)

The drying chamber exergy loss can be written as Eq. (18)

$${Ex}_{loss}={Ex}_{in}-{Ex}_{out}$$
(18)

The drying chamber’s exergy efficiency is expressed in Eq. (19) (Bhardwaj et al. 2019)

$${\eta }_{Ex,dc}=\frac{{Ex}_{out,dc}}{{Ex}_{in,dc}}$$
(19)

Sustainability analysis

The indicators of sustainability are influenced by the changes in the system’s exergy loss. IP, SI, and WER of the drying system can be expressed in Eqs. (20–22) (Ndukwu et al. 2017)

$$IP=\left(1-{\eta }_{Ex}\right){Ex}_{loss}$$
(20)
$$SI={\frac{1}{1-\eta }}_{Ex,d}$$
(21)
$$WER=\frac{{Ex}_{loss,dc}}{{Ex}_{i,dc}}$$
(22)

Environmental analysis

Before dryer fabrication, all the materials used for construction should be analyzed to know their environmental effects.

Energy payback period

The energy payback period of the ETSC drying system can be calculated by Eq. (23) (EL khadraoui et al. 2019)

$$Energy \ pay \ back \ period\left(EPBP\right)=\frac{EE\left(kWh\right)}{{E}_{O,T}\left(kWh/year\right)}$$
(23)

where EE and E0,T are the embodied energy and total energy output per year by the entire drying system.

CO 2 emissions

CO2 emissions omitted by the dryer can be calculated by using Eq. (24) (Eltawil et al. 2018)

$${CO}_{2} \ emision \ per \ year=\frac{EE\times 0.98}{{LT}_{d}}$$
(24)

where LTd is the overall dryer lifespan

CO 2 mitigation

The value of CO2 mitigation from the ETSC dryer is calculated by Eq. (25) (Singh and Gaur 2021)

$${CO}_{2} \ mitigation \ by \ the \ dryer=\left({E}_{O,T}\times {LT}_{d}-EE\right)\times 2.042$$
(25)

The carbon credit obtained with the ETSC dryer is calculated by Eq. (26) (Srivastava et al. 2021)

$$carbon \ credit \ earned={Net CO}_{2}\times Cost \ of \ mitigation$$
(26)

Economic evaluation

An economic evaluation of a drying system is an effective tool for designing a dryer for considering the cost parameters. In the present analysis, the life cycle saving method and payback period are considered for evaluating costs. In the life cycle saving method, initially, the value of the savings per day is evaluated, and then per year is calculated to dry the product (Singh et al. 2006) (Sethi and Dhiman 2020).

The cost of drying a fresh product (Cfd) can be calculated by Eq. (27)

$${C}_{fd}={C}_{fp}\times \frac{{m}_{fp}}{{m}_{dp}}$$
(27)

where Cfp, mfp, and mdp are the cost of the fresh product (undried product), the mass of the fresh, and dried samples.

The price for drying 1 kg of fresh Krishna tulsi leaves (Cds) is the sum of the price of the fresh leaves to the cost of dried leaves (Cfd) and the overall cost of the solar drying process (Cs), shown by Eq. (28).

$${C}_{ds}={C}_{fd}+{C}_{d}$$
(28)

The overall drying cost (Cs) is calculated from the cost of annualized drying (Can). The annual drying cost is calculated by Eq. (29).

$${C}_{an}={C}_{ac}+{C}_{m}-{V}_{a}+{C}_{rf}+{C}_{re}$$
(29)

where Cac is the annualized capital cost, Cm is the maintenance cost (assumed as 10% of the overall cost), Crf is the running fuel cost, Cre is the electricity cost, and Va is the salvage cost (assumed as 5% of the total cost).

The annualized capital cost (Cac) of a solar dryer can be estimated from the capital cost (Ccc) and the capital recovery factor (Crf), as shown by Eqs. (30 and 31) (Singh et al. 2021).

$${C}_{ac}={C}_{cc}+{C}_{rf}$$
(30)
$${C}_{rf}=\frac{k\times {\left(1+k\right)}^{n}}{{\left(1+k\right)}^{n}-1}$$
(31)

The unit cost of electrical energy to run the blower is calculated by Eq. (32)

$${C}_{re}={t}_{hr}\times {P}_{bl}\times {C}_{ele}$$
(32)

where thr is the blower running time in hours, Pbl is the power consumption value, and Cele is the electricity cost in units.

The total mass of Krishna tulsi leaves in the ETSC dryer per year (my) is expressed as Eq. (33)

$${m}_{y}={m}_{d}\times \frac{{t}_{sun}}{{t}_{d}}$$
(33)

where md is the mass of Krishna tulsi leaves dried per batch, tsun is the availability of the total sunny days for solar drying (assuming 200 clear sunny days in a year), and td is the total time taken for drying one batch.

The amount of savings for drying a single kg (Skg) of Krishna tulsi leaves is the price difference between the current market price (Cmp) of the dried product and the amount of the cost to dry in the solar dryer (Csd), which is given as Eq. (34).

$${S}_{kg}={C}_{mp}-{C}_{sd}$$
(34)

The amount of savings per day (Sd) in ETSC drying is calculated by the ratio between the amount of savings per batch (Sb) to the typical time for drying it (tt), expressed as Eq. (35).

$${S}_{d}=\frac{{S}_{b}}{{t}_{t}}=\frac{{{S}_{kg}\times m}_{d}}{{t}_{t}}$$
(35)

The amount of annual savings (Rk) to dry the product in the nth year is calculated by Eq. (36)

$${R}_{k}={R}_{d}\times {t}_{sun}\times {\left(1+{i}_{R}\right)}^{n-1}$$
(36)

where “iR” is the inflation rate.

The ETSC dryer payback period is expressed as Eq. (37)

$$N=\frac{In\left[1-\frac{{C}_{cc}}{{R}_{1}}\left(k-{i}_{R}\right)\right]}{In\left[\frac{{1+i}_{R}}{1+k}\right]}$$
(37)

where R1 is the savings during the operations in the first year.

Pressure loss in the air heater

It is important to find the ETSC pressure drop during the experiment. The air was sent to the ETSC inlet by forced convection. The pressure drop increases with the friction factor of the pipe. Smooth surface of the inner tube was used in this study as roughened tube increases pressure loss and it create obstacle to the flow. Pressure drop can be calculated by using the Darcy-Weisbach formula and expressed in Eq. (38)

$$\Delta P=\frac{{\rho fLv}^{2}}{2D}$$
(38)

The Reynolds number of the fluid flow in the air heater is calculated by using Eq. (39)

$$Re=\frac{\rho vD}{\mu }$$
(39)

For the laminar flow (Re < 2000), the friction factor can be expressed by Eq. (40)

$$f=\frac{64}{Re}=\frac{64\mu }{\rho vD}$$
(40)

By substituting the value of “f” in the Eq. (38), the pressure loss of the ETSC can be determined.

Determination of the convective heat transfer coefficient

The term convective heat transfer is an important parameter in the effectiveness of a drying system and helps in the design of an efficient dryer. The value depends on the product, size, drying time, and moisture content of the product to be dried (Kumar et al. 2023). Convective heat transfer coefficient (hc) of a drying system is represented by the following Eq. (41) (Lingayat et al. 2020) (Elbrashy et al. 2022).

$${h}_{c}=\frac{KNu}{l}$$
(41)

Let Qem be the heat used to evaporate the moisture, represented with Eq. (42) (Manchanda et al. 2019) (Barnwal and Tiwari 2008)

$${Q}_{em}={0.016h}_{c}\left[P\left({T}_{c}\right)-\lambda P\left({T}_{r}\right)\right]$$
(42)

where P (Tc) is the partial pressure of air at drying product; λ is the relative humidity of the air; P (Tr) is the partial pressure of air near the drying product.

By placing the value of hc in the Eq. (42)

$${Q}_{em}=0.016\frac{K \ Nu}{l}\left[P\left({T}_{c}\right)-\gamma P\left({T}_{r}\right)\right]$$
(43)

The amount of the moisture evaporation from the drying product can be calculated as

$${M}_{evaporation}=\frac{{Q}_{em}}{{h}_{fg}}{A}_{t}T$$
(44)

where A is the tray area in m2, t is time, and hfg is the latent heat value in kJ/kg K.

By substituting Qem in Eq. (44)

$${M}_{evaporation}=\frac{0.016 \ K \ Nu\left[P\left({T}_{c}\right)-\gamma P\left({T}_{r}\right)\right]}{{1h}_{fg}}{A}_{t}t$$
(45)

Let us assume

$$z=\frac{0.016K\left[P\left({T}_{c}\right)-\lambda P\left({T}_{r}\right)\right]}{{1h}_{fg}}{A}_{t}t$$
(46)

Then

$${M}_{evaporation}=zNu$$
(47)

Here

$$Nu=\frac{{M}_{evaporation}}{z}=C{\left[Re\times Pr\right]}^{n} \ \left(forced \ convection\right)$$
(48)
$$Nu=\frac{{M}_{evaporation}}{z}=C{\left[Gr\times Pr\right]}^{n} \ \left(natural \ convection\right)$$
(49)

Applying a logarithm to both sides

$$In \ Nu=n\times In \ \left(Gr\times Pr\right)+In \ \left[C\right]$$
(50)
$$In \ Nu=n\times In \ \left(Re\times Pr\right)+In \ \left[C\right]$$
(51)

The above equations are in linear equation y = mx + c form.

The constants C and n in the linear equation are evaluated by linear regression analysis

$$n=\frac{n\times \sum \left(XY\right)-\sum \left(X\right)\times \sum \left(Y\right)}{N\sum \left({X}^{2}\right)-{\left(\sum X\right)}^{2}}$$
(52)
$$C=\frac{\sum \left({X}^{2}\right)\times \sum \left(Y\right)-\sum \left(XY\right)\times \sum \left(X\right)}{N\times \sum \left({X}^{2}\right)-{\left(\sum X\right)}^{2}}$$
(53)

Results and discussions

Experimental conditions

Krishna tulsi leaves are dried in ETSC and OSD. The final MC of 12 ± 2% (db) of Krishna tulsi leaves is attained in 8 h in the ETSC dryer and 22 h in OSD. During the experimentation, the ambient temperature and solar radiation values with drying time are displayed in Fig. 4. Solar radiation is higher at 2 PM due to higher solar intensities and lesser at the initial stages of drying each day. The radiation is nearly the same on all three days of the experimentation, and the average solar intensity is 720 ± 20 W/m2. The ambient temperature on all three days ranges between 24 and 34 °C. The highest ambient temperature is found between 1 and 2 PM every day. The variation in ambient temperature might be due to changes in global solar radiation throughout the day. It can be observed from the figure that, with the increased solar intensity and ambient temperatures during the experimentation, the drying time of Krishna tulsi is reduced significantly.

Fig. 4
figure 4

Ambient temperature, solar radiation with drying time

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Temperatures at different locations, relative humidity of the air at ambient and in the dryer, and useful heat gain during the experiment are depicted in Fig. 5. The inlet temperatures of ambient air entering the collector range from 33 to 40 °C during the experiment. After that, the temperatures of the heated air coming out of ETSC vary between 65 and 68 °C, with the highest value of 82 °C when solar radiation is high. The inlet and outlet temperatures of the drying cabinet vary between 50–55 and 40–38 °C, respectively. The useful heat gain in the collector also depends on solar radiation and ranges between 100 and 560 W. The experiment ambient relative humidity ranges between 75 and 45% and is inversely affected by solar radiation. The relative humidity values in the drying chamber range from 40 to 26%; this may be due to moisture removal taking place continuously in the drying chamber.

Fig. 5
figure 5

(a) Temperatures at different locations and (b) relative humidities of air and useful heat gain during the experiment

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The change in the moisture ratio and the rate of drying of leaves with respective drying times in the ETSC and OSD are portrayed in Fig. 6.

Fig. 6
figure 6

Variation of (a) moisture ratio and (b) drying rate with the drying time

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Initially, the drying process of Krishna tulsi leaves occurs rapidly in both modes of drying, and then the process is carried out in the falling rate period. The maximum reduction in moisture ratio is observed in the first six hours of the drying period. Afterwards, the moisture ratio slowly decreases. With the progression of time, the desired moisture is obtained after 8 h of drying in the ETSC drying mode and 22 h of drying in OSD mode. The drying rate could significantly affect the quality and aesthetics of the drying product. The drying rate of Krishna tulsi leaves is calculated by Eq. (5) with experimental data. It can be seen from the figure that the drying rate is higher in the ETSC compared to the OSD. This occurs due to the higher uniform temperatures coming out of the ETSC, whereas no uniform temperatures are noted in the OSD.

Energy analysis of the drying system

ETSC collector thermal efficiency is between 42 and 75% with quick heat generation. This is due to the absorption of heat by the copper tubes from the aluminum fin and the glass tube. At higher temperatures, the efficiency of the collector is found high. The dryer’s efficiency is found as 0–46%. The efficiencies of the ETSC and drying chamber with the change in time are shown in Fig. 7. The starting value of 1 on the X-axis indicates 9 AM.

Fig. 7
figure 7

Change of efficiencies in the collector and the dryer with the time

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Exergy analysis of the drying system

ETSC and drying chamber exergy inflow, exergy outflow, and exergetic efficiency values are shown in Fig. 8. ETSC exergy inflow and outflow rates range between 200–1400 and 0–60 W, respectively. The drying chamber’s inflow and outflow values are 0–50 and 0–14 W, respectively.

Fig. 8
figure 8

Variation in inflow and outflow of exergy: (a) ETSC and (b) drying cabinet with the time

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The ETSC exergetic efficiency initially increases to 28%, and then decreases over time. The values of exergy loss vary between 300 and 1100 W. The exergy losses increase with drying time and range between 2 and 85% in the drying chamber. The exergetic efficiency of the drying system varies from 23 to 45%, and this is shown in Fig. 9. This parameter depends on the moisture content of the product to be dried. It is likely that some amount of energy is absorbed by dryer walls due to absorbancy and improper insulation in the dryer walls. The exergetic efficiency is found lower at the beginning and medium stages of drying due to the higher energy requirement for the drying process to remove high moisture levels. The higher efficiency is found in the last hours of the drying process due to the less energy requirement of the drying product.

Fig. 9
figure 9

Variation in exergy efficiency and exergy loss: (a) ETSC and (b) drying cabinet with the drying hours

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Exergy sustainability analysis of the drying system

Exergy sustainability parameters are useful parameters to improve dryer design more efficiently. A lower value of exergy losses indicates the dryer’s effectiveness. Exergy indices, namely, sustainability index, waste exergy ratio, and improvement potential, are calculated. The waste exergy ratio is an essential parameter in exergy analysis and indicates the loss of exergy per unit of input exergy. The sustainability index, the waste exergy ratio, and improvement potential range from 0–18, 10–1, and 0–10, respectively. Figure 10 shows how the sustainability indices changes with drying time. The sustainability index increases as the drying time lengthens, whereas the waste exergy ratio declines as the drying time increases. The dryer’s improvement potential is found through the change in solar radiation. It increases as solar radiation values increase and the other way around.

Fig. 10
figure 10

Change in the sustainability indices of the dryer with the drying time

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Environmental analysis of the drying system

The embodied energy of the ETSC dryer is calculated by taking the values of the energy density of different materials used to fabricate the dryer. The embodied energy required by the different materials to construct the solar dryer is given in Table 4. The dryer EPBT is estimated by the embodied energy value. Considering the dryer’s life span of 20 years, the energy payback time (EPBT) of the ETSC dryer is calculated as 1.2 years. Environmental analysis estimated that the energy payback period for the ETSC solar dryer is 2.21 years. For the developed dryer’s anticipated 20-year lifespan, the CO2 mitigation and earned carbon credit values are found as 13.2 tonnes and INR 10,894 to 43,576, respectively. With this evaluation, the ETSC-supported solar dryer would be an alternative option for the preservation and retention of various agricultural goods with ecological protection. Generally, with solar dryers, CO2 emissions decrease with the mitigation increase, and the system credit increases with the dryer life span.

Table 4 Embodied energy of the various materials employed in the proposed dryer
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Economic analysis of the ETSC dryer

The economic aspect of a solar dryer plays a significant role in the commercialization of the product. It reveals many economic factors for further use. The manufacturing cost of the entire drying system and the cost of drying the product should be lesser than the same dried product already available on the market. In some cases regardless of the product price, the quality of the dried product is much more significant when taken for medicinal use. An indirect solar dryer with ETSC is fabricated with a capital cost of 40,000 INR. Table 5 gives the price of the different materials/products used to make the dryer.

Table 5 Cost of the various devices/materials used in the dryer fabrication
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The price of the material purchased was the market price in Rourkela, Odisha, India. The inflation rate is assumed to be 7%, and the price of fresh and dried Krishna tulsi leaves is 30 and 250 INR, respectively. The annual drying cost of the product in the ETSC dryer is 2500 INR. The payback period is calculated as 0.4 years. The reported value to an ETSC-supported dryer is comparatively lower than thermal/electric dryers and found higher quality in the dried product than the products dried in OSD.

Pressure drop in the ETSC

Pressure drop and flow rate are interconnected. A constant mass flow rate of air was maintained throughout the experimental procedure. Reynolds number was calculated and the flow is laminar. From the experimental calculation, the pressure loss between the ETSC inlet and outlet is found to be 0.21 bar.

Convective heat transfer coefficient

Regression analysis was used to evaluate the constants in the convective heat transfer equations. By substituting the values of C and n, the value of hc can be evaluated. Table 6 lists the constants in the ETSC and OSD drying modes. Higher values of the convective heat transfer coefficient increase drying rates of the product. The average value of heat transfer coefficient in ETSC and OSD modes is 4.1 and 1.3 W/m2K respectively. The results are also in good agreement with those published by other researchers (Kumar et al. 2023) and (Sansaniwal et al., 2023). The change in convective heat transfer coefficient with drying time is shown in Fig. 11.

Table 6 Heat transfer coefficient values from the regression analysis
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Fig. 11
figure 11

Change in the convective heat transfer coefficient with the drying time

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Conclusion

An experimental investigation was carried out with the use of ETSC connected indirect solar dryer to dry Krishna tulsi leaves. Further energy, exergy, environmental, and economic analyses of the drying system were also performed. The research yielded the following conclusions;

  • A temperature of 40 °C is found higher in the collector outlet than the atmospheric temperature. The temperatures entering the drying chamber range between 60 and 70 °C, indicating that the device is capable the required drying temperatures

  • The developed ETSC dryer takes 8 h to dry Krishna tulsi leaves, whereas it takes 22 h in the OSD. According to the results, the ETSC dryer has a significantly shorter drying time than the OSD.

  • The exergy inflow and outflow of the ETSC and the drying chamber change from 200–1400 W, 0–60 W, and 0–50 W to 0–14 W, respectively. The ETSC and drying chamber exergetic efficiency vary from 0.6–4% and 2–85%, respectively. The overall drying process has an exergetic loss of between 0 and 40%.

  • The collector and dryer efficiencies vary from 42–75% and 0–18%, respectively, with solar radiation varying from 720 ± 20 W/m2. Dryer sustainability indices are calculated. The values of the sustainability index, waste exergy ratio, and improvement potential are 0–18, 10–1, and 0–10, respectively.

  • The value of the embodied energy of the fabricated ETSCS dryer is 349.874 kWh.

  • For the dryer’s anticipated 20-year lifespan, the CO2 mitigation and earned carbon credit values are 13.2 tonnes and INR 10,894 to 43,576, respectively. This parameter shows the adoption of solar dryers for environmental sustainability toward a greener world.

  • The payback period for the proposed dryer is estimated to be 0.4 years.

  • The developed dryer can be suitable for drying products that are sensitive to direct solar radiation. The results of the current research can broadly be useful in the applications of drying Krishna tulsi leaves in the pharma industry for use in the medical field.