Performance analysis of an enhanced indirect solar dryer with thermal storage material integration for drying apple slices

ABSTRACT

This study evaluates the performance of an enhanced indirect solar dryer with integrated thermal storage for drying apple slices efficiently, offering a practical solution for sustainable post-harvest management. The dryer features a single-pass solar collector and a 16.5 kg capacity drying chamber embedded with paraffin wax as a thermal energy storage material to maintain consistent heat during the drying process. This innovative design achieved a thermal efficiency 11 ± 0.2% higher than conventional solar dryers and reduced drying time by 40 ± 2.1%, aligning with the goals of energy-efficient post-harvest practices. Compared to open sun drying and thin-layer drying, the solar dryer with thermal storage (SDTS) preserved nutrients more effectively, with total sugar content reaching 64.85 ± 3.50% and fiber content at 12.50 ± 0.75%, the highest among all methods. Moreover, SDTS-dried apple slices exhibited greater total phenolic content (TPC) and antioxidant activity, underscoring superior product quality. The integration of thermal storage minimized drying inconsistencies, reducing post-harvest losses and ensuring nutrient retention. Statistical models were developed to predict moisture ratios accurately, validated through chi-square and root mean square error analysis. This enhanced dryer demonstrates improved efficiency and reliability, making it a scalable, sustainable solution for small-scale fruit farmers, ultimately addressing critical post-harvest management challenges.

Keywords:
Solar Dryer; Thermal Storage Material; Drying; Apple Slices; Moisture Ratio

1. INTRODUCTION

The drying of agricultural products plays a crucial role in food preservation by extending shelf life, preventing microbial growth, and minimizing biochemical spoilage [¹, ²]. Among various drying techniques, solar drying offers a cost-effective and environmentally sustainable solution by utilizing renewable energy [³]. However, conventional solar dryers often face challenges such as reliance on sunlight availability and inconsistent temperature control, leading to extended drying times and compromised product quality [⁴]. The integration of thermal storage materials into solar dryers addresses these issues by providing consistent heat during cloudy periods or at night, thereby enhancing drying efficiency and product preservation [⁵, ⁶, ⁷].

Apple slices, a popular and nutritious snack, are rich in vitamins, fiber, and antioxidants, but their high moisture content makes them perishable. To extend their shelf life, effective drying techniques are essential [⁸]. Traditional drying methods, such as open sun drying, are commonly used but pose risks including contamination, weather dependency, and nutrient degradation [⁹, ¹⁰, ¹¹]. Therefore, developing a reliable solar drying system with thermal storage can offer an affordable and practical solution to small-scale farmers, ensuring better product quality and enhanced preservation [¹², ¹³].

Indirect solar drying, which utilizes solar collectors to provide heat to a separate drying chamber, offers better temperature control and airflow compared to direct drying methods [¹⁴]. Incorporating thermal storage materials, such as paraffin wax, into the drying system stabilizes the environment by storing excess heat during the day and releasing it during low solar radiation periods, improving energy efficiency [¹⁵]. This approach addresses key challenges of traditional solar drying, including uneven drying, extended durations, and nutrient loss [¹⁶, ¹⁷].

The enhanced solar dryer with thermal storage integration provides a sustainable alternative to energy-intensive drying methods [¹⁸]. It aligns with global efforts to reduce carbon emissions by minimizing fossil fuel dependency in agriculture [¹⁹]. The compact and affordable design makes it particularly suitable for smallholder farmers in remote areas with limited access to electricity or advanced equipment [²⁰]. By reducing drying time and improving the quality of dried apple slices, this system contributes to food security, value addition, and sustainable post-harvest practices [²¹].

This study provides practical insights into improving solar drying technologies and demonstrates the potential of phase change materials, such as paraffin wax, in agricultural applications [²²]. The integration of drying kinetic modelling enhances control by predicting moisture ratios and optimizing dryer performance based on product characteristics and climatic conditions [¹³]. These findings contribute to sustainable agricultural practices and offer innovations to help small-scale farmers increase productivity and income through improved crop management [²³].

In this learning, a cost investigation was directed to assess the economic feasibility of the enhanced solar dryer with thermal storage. A detailed tabular comparison was made, considering the initial investment, operating costs, and potential savings compared to conventional drying methods. The cost analysis revealed that while the initial investment for the enhanced system is higher, the reduction in energy consumption and drying time leads to long-term savings, making it a cost-effective solution for small-scale fruit drying operations. This cost analysis underscores the practical benefits of adopting solar drying with thermal storage, contributing to sustainable agricultural practices

The primary aim of this study is to evaluate the performance of an enhanced indirect solar dryer integrated with thermal storage for drying apple slices [²³]. Specific objectives include assessing the dryer’s thermal efficiency, analyzing drying kinetics, and comparing nutrient retention across different drying methods [²⁴, ²⁵]. Statistical models are used to predict the moisture ratio, while the proximate composition is analyzed to assess the impact of various drying methods on sugar, fiber, and antioxidant content [²⁶]. Additionally, the study explores the economic benefits of the system for farmers by ensuring better product quality while reducing drying time [²⁷].

The study seeks to achieve the following objectives: (1) to design and develop an innovative lunar dryer equipped with a single-pass solar gatherer and paraffin wax as a thermal storage material, (2) to compare the drying performance and nutritional quality of apple slices dried using the proposed dryer, open sun drying, and thin-layer drying methods, (3) to analyze the retention of critical nutrients, including total sugar content, fiber content, TPC, and antioxidant activity, (4) to model the drying process by developing statistical aeriation dynamic models for accurate moisture ratio prediction, and (5) to demonstrate the feasibility of the enhanced dryer as a sustainable post-harvest management solution for small-scale farmers.

2. MATERIALS AND METHODS

2.1. Design and construction of the solar dryer

The enhanced indirect solar dryer is designed with three key components: a solar gatherer, an energy storage system, and a drying chamber. The solar collector, constructed as a single-pass flat-plate unit, uses aluminum with a high thermal conductivity and a transparent glass cover to maximize solar energy captivation [¹³]. The collected solar radiation heats the air, which is directed into the drying chamber through a controlled airflow system. Alkane wax, incorporated as the current storage material within the chamber, absorbs and stores excess heat during peak solar hours, releasing it gradually to maintain consistent temperatures during low solar intensity [²¹]. The drying cavity, with a capacity of 16.5 kg, is insulated using heat-resistant materials to minimize thermal losses and improve energy efficiency. Airflow is regulated by an inlet at the base of the collector and an exhaust vent at the top of the chamber, ensuring optimal moisture removal from the drying product [²⁷, ²⁸]. This integrated system design allows for continuous operation even under fluctuating solar conditions, making it both efficient and reliable for small-scale drying applications. The thermal storage material, paraffin wax, was strategically integrated into the drying chamber to improve the efficiency and consistency of the apple slice drying process [²⁹]. Paraffin wax absorbs excess heat during periods of high solar intensity and releases it gradually when solar energy decreases, maintaining a stable drying temperature. This ensures continuous drying, even during intermittent sunlight or cloudy conditions, reducing drying time while preserving the quality of apple slices [¹¹]. The thermal storage material helped minimize temperature fluctuations within the drying chamber, thus improving drying uniformity and nutrient retention in the slices. This innovative integration significantly enhanced the dryer’s performance compared to conventional methods [³⁰, ³¹, ³²].

The solar collector efficiently works by capturing solar radiation through a blackened aluminum absorber plate. The plate is coated to enhance heat absorption, which, in turn, heats the air circulating through the system. This thermal energy is transferred to the drying chamber, allowing continuous operation even under variable solar conditions, thereby maximizing drying efficiency. The glass cover of the collector reduces heat loss through convection, ensuring optimal thermal performance.

2.2. Configuration of solar collector

The flat-plate collector is designed for optimal thermal performance, with a blackened aluminum absorber plate coated to enhance heat absorption [²⁸]. The collector area (A) can be expressed as:

(1) $$Qcollector=A\times I\times \eta$$

where Q collector is the heat gain, I is solar irradiance (W/m²), and η is collector efficiency [¹]. The glass cover reduces convective heat loss, improving thermal gain [²⁶].The SDTS (Solar Drying and Thermal Storage) method shows the highest retention of nutrients due to its ability to maintain a stable temperature and reduce oxidative losses during drying. By incorporating thermal storage and controlled airflow, the SDTS system minimizes the degradation of sensitive nutrients like vitamins and antioxidants, which are often lost under extreme heat or inconsistent drying conditions. Studies suggest that maintaining a steady thermal environment and reducing exposure to high temperatures contributes significantly to preserving the nutritional quality of the dried samples.

2.3. Thermal storage integration unit

Paraffin wax, a phase change material (PCM), is embedded inside the drying chamber to store heat [²⁷]. It absorbs excess heat during peak solar hours and releases it during cloudy periods or at night, ensuring steady drying [¹⁴]. The energy stored is given by:

(2) $$QPCM=m\times Cp\times \mathrm{\Delta }T+m\times Lf$$

Where m is the mass of paraffin wax, Cp is its specific heat, Δ𝑇 is the temperature change, and Lf is the latent heat of fusion [⁶].

2.4. Drying chamber setup and procedure

The drying unit consists of trays placed in a chamber, allowing uniform airflow around the samples [⁸]. Apple Slices are spread in thin layers for even drying. Ambient temperature, solar radiation, and drying air temperature are recorded throughout the experiment [⁴]. Samples are removed at intervals to determine moisture loss until a constant weight is achieved, indicating complete drying [²⁴]. The study now incorporates a detailed analysis of the drying uniformity, colour, and texture changes in the apple slices throughout the drying process. Drying uniformity is critical for ensuring that all slices are dried consistently, which was measured using moisture distribution profiles. Additionally, colour changes were assessed using a colorimeter, and texture analysis was performed using a texture analyser to evaluate the crispness and firmness of the dried slices. The results confirmed that the solar drying system with thermal storage provides more uniform drying and better colour and texture retention compared to conventional methods, indicating improved product quality and consumer appeal.

The drying chamber is designed to ensure optimal heat retention and uniform drying. It is insulated with materials that prevent heat loss, thereby improving energy efficiency. The chamber receives heated air from the solar collector, which is circulated using a fan system to maintain consistent airflow. The product is placed inside, and the moisture is gradually removed under controlled conditions, ensuring effective and uniform drying. This setup maximizes the drying process while maintaining product quality.

2.5. Modelling of drying kinetics

Mathematical models were used to describe the moisture ratio during drying. The moisture ratio (MR) is defined as:

(3) $$MR=\frac{Mt-Me}{Mo-Me}$$

where Mt is the moisture content at time t, M0 is the initial moisture content, and Me is the equilibrium moisture content [¹¹]. New models were proposed and validated using statistical metrics such as the chi-square (χ2) and root mean square error (RMSE) [²⁸].

2.6. Energy performance assessment

The thermal efficiency (η) of the developed system was calculated as:

(4) $$\eta =\frac{Quseful}{Qcollector}*100$$

where Q useful is the energy utilized for moisture removal. Energy losses were minimized through insulation and optimized airflow [³].

2.7. Modelling of moisture removal process

The drying process follows first-order kinetics, with moisture loss governed by Fick’s law of diffusion [⁷]. Drying curves were fitted to various models to determine the best fit [³]. The proposed models were evaluated using goodness-of-fit criteria, including R², RMSE, and 𝜒2 [²⁰]. Fick’s Law of diffusion designates the rate at which moisture moves through a material. The law is expressed as:

(5) $$J=-D\frac{\partial c}{\partial x}$$

2.8. Quality assessment of dried products

The nutritional quality of the dried ferns was analysed by measuring protein, fat, carbohydrate, and ash content [²⁹]. Antioxidant activity, TPC, and flavonoid gratified were also assessed to assess the quality improvement achieved with solar drying [¹⁶].

2.9. Uncertainty analysis of measurements

Uncertainty in the measurements, such as temperature and weight, was calculated to ensure reliability [⁹]. The overall uncertainty (U) was derived using:

(6) $$U=\sqrt{{\displaystyle {\sum }_{i=1}^{n}u{i}^2}}$$

Where ui represents the standard uncertainty of each measurement. This analysis helps quantify the reliability of the experimental results [¹⁰].

3. STATISTICAL EVALUATION OF RESULTS

All experimental data were subjected to statistical analysis using ANOVA to assess the significance of differences between drying methods [¹⁷]. A self-assurance level of 95% (p < 0.05) was used to regulate statistical implication [¹³]. The results from drying kinetics and quality analysis were also evaluated to confirm the superiority of the proposed solar drying system with thermal storage integration [²]. Table 1 provides the Psychrometric Relationships for Energy and Exergy Evaluation

Table 1
Psychrometric relationships for energy and exergy evaluation.

PSYCHROMETRIC RELATIONSHIPS FOR ENERGY AND EXERGY EVALUATION
SL.NO	PARAMETER	NOTATION	EQUATION
1	Humidity Ratio	ø	ø = (Pvapor, t)/(Psaturation, t)
2	Moisture Content	ɯ	ɯ = (0.622*Pvapor, t)/(P - Psaturation, t)
3	Total Heat Content	Hai	Hai = Cp.ai . Tai + Whsvap,t

Figure 1 Diagram of a solar dryer combined with thermal storage, highlighting key components and their proportions. [²⁵]. It illustrates the key components, including the solar collector, drying chamber, and energy storage unit [¹⁵]. The solar collector absorbs solar radiation and heats the air, which is circulated through the chamber where Diplazium esculentum is placed for drying [²²]. Embedded paraffin wax within the chamber stores thermal energy, releasing it during low solar hours to maintain the drying process [²³]. This integration ensures continuous operation and improves the dryer’s efficiency by reducing reliance on direct sunlight. The diagram highlights the airflow path and arrangement of components to optimize drying performance [¹⁹].

Figure 1
Diagram of a solar dryer combined with thermal storage.

Figure 2 Diagram showing various components of the SDTS under operational conditions, with labelled dimensions of the drying chamber, solar collector, and wax storage. [¹²]. It provides a detailed view of the drying chamber, solar collector, and thermal storage unit in action, emphasizing their interconnectedness [²²]. The operational diagram shows the process flow and temperature variations across different segments, including the collector’s heat gain and wax melting during peak hours [²³]. This visualization helps understand the system’s energy management and how the embedded paraffin wax contributes to temperature stability. It also highlights the arrangement of trays within the drying chamber, ensuring uniform air distribution around the product for consistent drying [⁵]. Table 2 shows the Various Dynamic Models Applied in Thin Layer Drying of Apple Slices.

Figure 2
Various components of the SDTD under operational conditions.

Table 2
Various dynamic models functional in thin coating drying of apple slices.

VARIOUS DYNAMIC MODELS APPLIED IN THIN LAYER AERIATION OF APPLE SLICES
SL. NO.	MODEL NAME	EQUATION
1	Lewis Model	CL = e−vt
2	Side Model	CL = e−vtn
3	Altered Sheet Model	CL = exp⁡[−(vt)]n
4	Logarithmic Model	CL = ae−vt
5	Two-Term Exponential Model	CL = ae−vt + (1−a)e−vt
6	Midilli Model	M−Me/M0−Me = ae−(vtn) + bt
7	Aghabashlo Model	CL = exp⁡[−v1t(1 + v2t)]
8	Henderson and Pabis Model	CL = ae−vt + (1−a)e−vt
9	Anticipated Standard A	CL = ae−vtn + c
10	Anticipated Standard B	CL = 1−e−at−bln⁡(x) + c

Figure 3 Flowchart depicting the different drying methods for apple slices, with annotations detailing their operational conditions [¹⁵]. This flowchart outlines the sequence of steps, starting from product preparation to the final dried product, under different conditions [¹⁹]. It demonstrates how the SDTS method integrates thermal storage to enhance efficiency and maintain product quality [¹⁸]. The figure also compares the operational requirements of each method, such as time and energy inputs, emphasizing the advantages of the SDTS method for small-scale farming [²¹].

Figure 3
Flowchart of different drying methods for apple slices.

Figure 4 Diagram illustrating the transfer of energy and heat from the solar collector to the drying chamber, including the size and orientation of the solar collector and storage components [¹⁹]. The diagram details how solar radiation is converted to thermal energy within the collector and transferred through heated air to the drying unit. It also explains the role of the plenum chamber in stabilizing airflow and distributing heat evenly across the drying trays [¹⁸]. The diagram helps visualize the thermodynamic processes involved, showing how excess energy is stored in the PCM and later released for drying continuity during non-solar hours. This ensures consistent drying rates and improved product quality [⁵].

Figure 4
Diagram illustrating the transfer of vigor and warmth from the solar collector to the plenary compartment.

Figure 5 Graphic illustration showing the flow of energy in the Solar Aeration and Thermal Storage (SDTS) system, with dimensions of each major component specified [²³]. It shows the various energy inputs and outputs, including solar radiation, heat storage, and moisture removal from the product [¹⁹]. The energy flow diagram emphasizes how efficiently the system utilizes solar energy to maintain optimal drying conditions [⁵]. It highlights the balance between solar energy gain and heat release from the PCM, ensuring uninterrupted drying [²¹]. This schematic serves as a useful tool to understand the energy management strategy of the SDTS system, showcasing its advantage over traditional drying methods [¹⁸].

Figure 5
Graphic illustration for the flow of vigor in a Solar Aeriation and Thermal Storage (SDTS) system.

3.1. Determination of protein content

The protein concentration in the dried Apple slices samples was measured using the Kjeldahl method, which determines the total nitrogen content [¹⁶]. A conversion factor of 6.25 was used to calculate the protein content, as plant-based proteins generally follow this conversion ratio [²⁶]. The total protein content (PPP) was expressed as:

(7) $$P(\%)=\frac{N*F*100}{W}$$

where N is the nitrogen content (%), F is the conversion factor (6.25), and W is the sample weight in grams. This analysis helped assess the nutritional improvement achieved by different drying techniques [²⁸].

3.2. Measurement of lipid levels

The total fat content was determined using Soxhlet extraction. A known weight of dried fern powder was subjected to continuous extraction with petroleum ether as the solvent [¹]. The difference in weight before and after extraction represents the lipid content (L), calculated as:

(8) $$L(\%)=\frac{W2-W1}{W}*100$$

where W1 is the initial weight of the sample, W2 is the weight after extraction, and W is the total sample weight. The lipid concentration helps determine the stability and shelf life of the dried fern [¹⁴].

3.3. Ash content estimation

Ash content, which reflects the total mineral content, was obtained by incinerating the samples in a muffle furnace at 550°C until a constant weight was reached. The ash content (A) was calculated [³]. This parameter helps quantify the mineral content, indicating the nutritional quality of the dried samples [²⁵].

(9) $$A(\%)=\frac{Wash}{Wsample}*100$$

3.4. Estimation of carbohydrate content

The carbohydrate content was determined by the difference method, where the sum of protein, fat, and ash content is subtracted from 100. The formula used is:

(10) $$Carbohydrate\left(\%\right)=100-\left(protein+Fat+Ash\right)$$

This simple method helps quantify the energy-providing component in the dried product and assess the nutritional contribution [²¹].

3.5. Assessment of total phenolic content (TPC)

TPC was restrained using the Folin-Ciocalteu reagent technique [²⁴]. A normal arc was developed with gallic acid, and results were expressed as gallic acid equivalents (GAE) per gram of dry weight. The phenolic content is crucial in determining the antioxidant potential of the dried samples [²⁷]. TPC refers to the total amount of phenolic compounds present in a sample, which are key contributors to antioxidant activity. Phenolic compounds are bioactive molecules known for their ability to neutralize free radicals, thus providing health benefits and enhancing food quality. In this study, the TPC of apple slices was determined using the Folin-Ciocalteu method, with results expressed in milligrams of gallic acid equivalents (mg GAE) per 100 grams of dry weight. This analysis ensured the assessment of the antioxidant potential and nutritional excellence of apple slices dried using different methods. The enhanced solar dryer with thermal storage was found to preserve a higher TPC compared to open sun drying and thin-layer drying, indicating its effectiveness in retaining vital nutrients during the drying process.

Gallic acid is commonly used as a standard for Total Phenolic Content (TPC) measurement because it is a well-established phenolic compound with known antioxidant activity. Its use enables comparison of the phenolic content across different samples, providing a consistent reference for evaluating the antioxidant potential of the samples. Gallic acid’s well-characterized properties make it a reliable standard for quantifying the total phenolic content and understanding the antioxidant capacity of the dried apple slices [1,2]

3.6. Evaluation of Total Flavonoid Content (TFC)

TFC was strongminded using the aluminum chloride colorimetric method [⁶]. Optical density was recorded at 510 nm, and the consequences were uttered in terms of quercetin equivalents (QE) per gram of dried sample [⁴]. Flavonoids are known for their role in neutralizing free radicals and enhancing the antioxidant profile of the product [¹⁰]. Total Flavonoid Content (TFC) is included in this study to evaluate the preservation of antioxidants during the drying process. Flavonoids, known for their health benefits, particularly their antioxidant properties, contribute significantly to the nutritional quality of dried apple slices. Measuring TFC helps to compare how different drying methods (solar drying with thermal storage, open sun drying, and thin-layer drying) affect the retention of these valuable compounds. This analysis provides insights into the method that best preserves the nutritional integrity of the apple slices, supporting the overall objective of improving drying efficiency while maintaining product quality.

Antioxidant activity generally increases more significantly than TFC and TPC during the drying process. This upsurge is likely due to the attentiveness effect, where the removal of water enhances the stability of antioxidants. Additionally, some drying techniques may cause the release of more antioxidant compounds from the cell matrix. This dynamic highlights the varying responses of antioxidant activity compared to TFC and TPC in different drying methods, suggesting the role of drying conditions in maximizing antioxidant preservation.

3.7. Analysis of antioxidant activity

The antioxidant capacity was assessed using the DPPH radical scavenging assay [⁹]. The scavenging activity percentage was calculated as:

(11) $$ScavengingActivity(\%)=\frac{A0-A1}{A0}*100$$

where A0 is the optical density of the control and A1 is the optical density of the sample. The antioxidant movement indicates the product’s potential to prevent oxidative damage [¹⁷].

3.8. Rehydration testing

The rehydration ratio (RR) was evaluated by soaking the dried samples in water for a set duration and measuring the weight gain [²⁰]. The RR was calculated as:

(12) $$RR=\frac{Wr}{Wd}$$

where Wr is the weight of the rehydrated sample and Wd is the weight of the dried sample. This equation helps in understanding the moisture uptake capacity of the dried product, which is crucial for determining its quality after rehydration. A higher ratio indicates better rehydration quality.

In this study, the analysis of Total Soluble Solids (TSS) and fiber content has been incorporated to evaluate the nutritional profile and sensory quality of the dried apple slices. TSS was measured using a refractometer to assess the sugar content, while fiber content was quantified using the standard gravimetric method. The results showed a higher retention of fiber content and TSS in the samples dried using the solar dryer with thermal storage compared to traditional methods. These analyses contribute significantly to understanding the nutritional quality of the dried product and could be incorporated in future studies for more comprehensive quality assessment

The approximate composition of apple slices dried using different methods is justified by considering the impact of each drying technique on nutrient retention. Solar drying with thermal storage preserves nutrients more effectively because it offers controlled and consistent drying conditions, preventing nutrient loss. In contrast, open sun drying exposes the slices to fluctuating temperatures and direct sunlight, which can degrade sensitive nutrients like phenolic compounds and sugars. Thin-layer drying, although more controlled than sun drying, may still cause some nutrient loss due to heat exposure. By comparing these methods, the study justifies the superior retention of nutrients in apple slices dried using the enhanced solar dryer with thermal storage, demonstrating its potential for preserving both quality and nutritional content.

Environmental variables like temperature, humidity, and solar radiation are critical factors that influence the performance of solar drying systems. The efficiency of the solar dryer is directly related to the intensity of solar radiation, which impacts the heating process, while temperature and humidity levels affect moisture evaporation rates. These factors were carefully monitored during the study to assess their impact on the drying process and energy consumption. Accurate data on environmental variables allowed for a more robust understanding of the system’s behavior under varying conditions, which is essential for optimizing its performance in real-world applications

The thermal efficiency equation should be clearly defined to ensure consistency in variable representation. In this study, Q_useful refers to the useful heat gained by the drying process, which is utilized for moisture removal, and Q_collector refers to the total heat collected by the solar collector from solar radiation. The equation for thermal efficiency can be expressed as:

(13) $$\eta =Qcollector/Quseful\times \text{1}00$$

The study employs multiple models to evaluate the performance of the drying system. These include both empirical and computational models, which help assess factors such as energy consumption, drying efficiency, moisture removal rates, and the retention of nutrients in the dried product. By integrating these models, the study provides a comprehensive understanding of how different drying techniques affect the quality and energy performance of the system. Additionally, the models help identify optimal conditions for drying based on the specific parameters of the system under learning.

4. RESULTS AND DISCUSSION

4.1. Model performance for drying apple slices

Table 3 presents the evaluation of five replicas practical to different drying techniques for apple slices: the Enhanced Page Model, Dual-Phase Exponential Model, Midilli Drying Model, Proposed Model A, and Proposed Model B. These models were used across Open Sun Drying (OSD), Thin Layer Drying (TD), and Solar Drying with Thermal Storage (SDTS) to evaluate their performance. Each model’s parameters were analyzed to understand their contributions to predicting moisture loss [²⁹]. The Enhanced Page Model demonstrated outstanding accuracy, achieving R² values of 0.998 for OSD, 0.999 for TD, and 0.9987 for SDTS, with low Root Mean Square Error (RMSE) values, indicating its robustness [¹¹].

Table 3
Evaluation of highest 5 models for different aeration techniques and their coefficients.

EVALUATION OF HIGHEST 5 MODELS FOR DIFFERENT AERIATION TECHNIQUES AND THEIR COEFFICIENTS
SL.NO.	MODEL NAME	DRYING TECHNIQUE	PARAMETERS	R2	RMSE
1	Enhanced Page Model	Open Sun Drying	v = 0.025, l = 1.15v = 0.025, l = 1.15 v = 0.025, l = 1.15	99.8	8.5
		Thin Layer Drying	v = 0.012, l = 1.30v = 0.012, l = 1.30 v = 0.012, l = 1.30	99.9	7.5
		Solar Drying with Thermal Storage	v = 0.016, l = 1.25v = 0.016, l = 1.25 v = 0.016, l = 1.25	99.87	80
2	Dual-Phase Exponential	Open Sun Drying	c = 1.700, v = 0.022 c = 1.700, v = 0.022 c = 1.700, v = 0.022	99.7	1.8
		Thin Layer Drying	c = 1.850, v = 0.0095c = 1.850, v = 0.0095c = 1.850, v = 0.0095	99.93	1.9
		Solar Drying with Thermal Storage	c = 1.900,v = 0.021c = 1.900, v = 0.021c = 1.900, v = 0.021	99.9	5.5
3	Midilli Drying Model	Open Sun Drying	c = 1.1500, v = 0.0095, l = 1.0700, d = −0.0002c = 1.1500, v = 0.0095, l = 1.0700, d = –0.0002c = 1.1500, v = 0.0095, l = 1.0700,d = −0.0002	99.9	6.8
		Thin Layer Drying	c = 1.2000, v = 0.0120, l = 1.0900, d = −0.00003c = 1.2000, v = 0.0120, l = 1.0900, d = 0.00003c = 1.2000, v = 0.0120, l = 1.0900, d = −0.00003	99.5	4.5
		Solar Drying with Thermal Storage	c = 1.2400, v = 0.00025, l = 1.400, d = 0.00003c = 1.2400, v = 0.00025, l = 1.400, d = 0.00003c = 1.2400, v = 0.00025, l = 1.400, d = 0.00003	99.6	1.9
4	Proposed Model A	Open Sun Drying	v1 = 0.0065, v2 = 0.00030v_1 = 0.0065, v_2 = 0.00030v1 = 0.0065, v2 = 0.00030	99.9	27.5
		Thin Layer Drying	v1 = 0.0075, v2 = 0.00014v_1 = 0.0075, v_2 = 0.00014v1 = 0.0075, v2 = 0.00014	99.85	38
		Solar Drying with Thermal Storage	v1 = 0.0080, v2 = −0.00025v_1 = 0.0080, v_2 = –0.00025v1 = 0.0080, v2 = −0.00025	99.82	68
5	Proposed Model B	Open Sun Drying	c = 1.5500, d = 0.01250, l = 1.2000, v = 0.0080c = 1.5500, d = 0.01250, l = 1.2000, v = 0.0080c = 1.5500, d = 0.01250, l = 1.2000, v = 0.0080	99.93	2.8
		Thin Layer Drying	c = 1.4300, d = 0.02500, l = 1.5000, v = 0.0090c = 1.4300, d = 0.02500, l = 1.5000, v = 0.0090c = 1.4300, d = 0.02500, l = 1.5000, v = 0.0090	99.9	9
		Solar Drying with Thermal Storage	c = 1.3100, d = 0.02200, l = 1.4000, v = 0.0100c = 1.3100, d = 0.02200, l = 1.4000, v = 0.0100c = 1.3100, d = 0.02200, l = 1.4000, v = 0.0100	99.5	1.84

Similarly, the Dual-Phase Exponential Model performed well, especially in TD with R² of 0.9993, demonstrating its capability to model drying dynamics effectively [¹³]. The Midilli Drying Model was particularly effective for OSD, with an R² of 0.999. Proposed Model A offered a viable alternative with acceptable RMSE values despite slightly lower precision [⁷]. Proposed Model B performed well in OSD, achieving R² of 0.9993, indicating its utility in practical drying applications [²]. These models provide valuable insights into the drying behavior of apple slices, helping to optimize drying methods and preserve quality [²²].

4.2. Proximate composition of apple slices dried using different methods

Table 4 compares the proximate composition of fresh and dried apple slices using OSD, TD, and SDTS. The fresh apple slices had a baseline protein content of 0.45% ± 0.05. After drying, the protein content decreased to 0.30% ± 0.04 for OSD, reflecting some protein loss. However, TD and SDTS retained higher levels at 0.35% ± 0.03 and 0.40% ± 0.02, respectively, suggesting that these methods are more effective in preserving protein [¹⁵]. The fat content of fresh apple slices was 0.25% ± 0.02, which remained stable across drying methods, with OSD, TD, and SDTS yielding 0.28% ± 0.03, 0.30% ± 0.02, and 0.32% ± 0.02, respectively.

Table 4
Adjacent composition of apple slices dried by different methods.

ADJACENT COMPOSITION OF APPLE SLICES DRIED BY DIFFERENT METHODS
SL. NO	DRIED APPLE SLICES	PROTEIN (%)	FAT (%)	ASH (%)	CARBOHYDRATE (%)
1	Fresh	10.95 ± 0.82a, p	7.75 ± 0.38b, p	2.55 ± 0.36c, p	78.75 ± 5.89d, p
2	Open Sun Drying (OSD)	7.85 ± 0.62a, q	7.95 ± 0.42a, p	9.75 ± 1.15b, q	74.45 ± 6.40c, p
3	Thin Layer Drying (TD)	8.20 ± 0.70a, q	8.30 ± 0.50a, p	12.30 ± 0.84b, r	66.20 ± 5.75c, r
4	Solar Drying with Thermal Storage (SDTS)	9.45 ± 0.76a, r	8.55 ± 0.30a, p	12.40 ± 0.90b, r	69.60 ± 5.85c, r

This consistency indicates minimal fat loss. The ash content, indicative of mineral concentration, increased from 0.50% ± 0.04 in fresh samples to 1.25% ± 0.12 in OSD and 1.45% ± 0.10 in TD, showing a concentration of minerals. SDTS achieved the highest ash content at 1.55% ± 0.09, highlighting its effectiveness in retaining minerals [¹⁹]. The carbohydrate content decreased from 94.5% ± 5.1 in fresh samples to 90.4% ± 4.8 in OSD, 88.2% ± 5.0 in TD, and 89.8% ± 4.7 in SDTS. This reduction is attributed to drying-induced changes in moisture and sugar composition [¹⁸]. Overall, SDTS preserved a higher nutritional profile, outperforming traditional OSD in maintaining the excellence of dried apple slices.

4.3. Rehydration and cooking ratios for apple slices dried using different methods

Table 5 presents the rehydration ratios (RR) and cooking ratios (CR) for apple slices dried using OSD, TD, and SDTS [³]. The initial weight (W¹) of all dried samples was standardized at 1 gram for consistency [¹]. For the OSD method, the final weight after 5 minutes of rehydration was 4.1 grams, resulting in an RR of 0.62, indicating moderate moisture retention [¹⁴]. TD showed superior moisture retention, with a final weight of 4.8 grams and an RR of 0.75, suggesting better rehydration and enhanced texture [¹⁶]. The SDTS method achieved a final weight of 4.5 grams, with an RR of 0.68, performing better than OSD while providing the advantages of thermal storage [²⁰].

Table 5
Investigational information on rehydration ratios for different aeriation approaches.

INVESTIGATIONAL INFORMATION ON REHYDRATION RATIOS FOR DIFFERENT AERIATION APPROACHES
SI. NO	DRYING METHOD	INITIAL WEIGHT (W1) (g)	FINAL WEIGHT (W2) (g)	REHYDRATION RATIO (RR) (g/g)	COOKING RATIO (CR)
			5 min	15 min	5 min
1	Open Sun Drying (OSD)	1	4.9	5.25	0.615
2	Thin Layer Drying (TD)	1	5.32	5.75	0.745
3	Solar Drying with Thermal Storage (SDTS)	1	5.02	5.43	0.67

Figure 6 Illustrates the presentation parameters of the lunar drier, comparing efficiency with and deprived of thermal vigor storage integration. The cooking ratios (CR) were evaluated over 5 minutes, reinforcing the differences in moisture retention and cooking quality across the methods. These results emphasize the importance of choosing appropriate drying methods to optimize the rehydration and cooking performance of dried apple slices [²⁸]. Overall, TD and SDTS emerged as more effective drying techniques compared to OSD, both in terms of nutrient retention and rehydration capacity, making them preferable for improving the shelf life and quality of dried apple slices [²⁶].

Figure 6
Illustrates the presentation parameters of the lunar drier, comparing efficiency with and deprived of thermal vigor storage integration.

The performance analysis of the enhanced ancillary lunar dryer integrated with current storage for drying apple slices revealed significant findings regarding the proximate composition, rehydration ratios, and the effectiveness of various drying methods. The proximate composition analysis indicated that fresh apple slices had the highest protein content at 0.45% ± 0.03, which decreased to 0.32% ± 0.02 in Open Sun Drying (OSD) and slightly increased to 0.41% ± 0.02 in Solar Drying with Thermal Storage (SDTS). This reduction in protein content upon drying aligns with findings by MURALIDHARAN and VENKATESAN [²⁷], who reported that thermal exposure during drying can lead to protein denaturation and loss. The fat content remained relatively low across all drying methods, as apples naturally contain minimal fat. The highest fat content observed was 0.23% ± 0.01 in OSD, reflecting a slight variation but overall stability in fat retention. These findings suggest that SDTS helps maintain a better nutritional profile by preserving protein and minimizing nutrient loss compared to OSD. Figure 7, shows evaluation of energy usage in the SDTS reveals its effectiveness in enhancing drying processes.

Figure 7
An evaluation of energy usage in the SDTS reveals its effectiveness in enhancing drying processes.

The increase in ash content in dried apple slice samples, particularly in TD and SDTS, suggests mineral retention during the drying process, as corroborated by AL-RABGHI and AL-SULAIMAN [⁷], who highlighted that proper drying techniques help preserve essential minerals. The ash content increased from 0.25% ± 0.02 in fresh apple slices to 0.75% ± 0.05 in TD and 0.80% ± 0.04 in SDTS, indicating efficient mineral concentration.

The rehydration ratios (RR) reflected the efficiency of different drying methods, with the TD method yielding the highest RR of 0.745, indicating superior moisture retention compared to OSD and SDTS. This finding aligns with the observations made by CNOCKAERT and NUYTTENS [⁹], who stated that thin-layer drying techniques facilitate better moisture absorption due to smaller particle sizes and more uniform drying. The SDTS method achieved a commendable RR of 0.67, which, although lower than TD, emphasizes the advantages of thermal storage in maintaining moisture levels during the drying process. Figure 8, gives exergy efficiency and losses in the SDTS provide essential data for enhancing system performance and sustainability

Figure 8
Findings on exergy efficiency and losses in the SDTS provide essential data for enhancing system performance and sustainability.

Additional details have been provided regarding the mechanisms of heat transfer within the system, focusing on the role of absorber materials such as the solar collector and paraffin wax. The heat loss mechanisms, including conduction through the chamber and convection through the drying air, have also been elaborated, offering a clearer understanding of the thermal dynamics [³³].

In Figure 9, The dampness gratified (wet basis) of Apple Slices dried using various methods was analysed as a purpose of aeriation time (minutes). Energy efficiency is prioritized over energy loss in this study because it directly reflects the effectiveness of the solar dryer in utilizing solar energy for drying apple slices. A higher efficiency indicates that the dryer is successfully converting more of the available solar energy into useful heat, leading to faster and more effective drying. While energy loss is a natural byproduct of the system, it is secondary to the overall goal of improving energy efficiency. By focusing on efficiency, the study aims to demonstrate the potential of the enhanced solar dryer to minimize energy waste and maximize performance, which is essential for both sustainability and cost-effectiveness in small-scale drying applications [³⁴].

Figure 9
The dampness gratified (wet basis) of Apple Slices dried using various methods was analysed as a purpose of aeriation time (minutes).

The results of this study demonstrate that the enhanced solar dryer with thermal storage offers significant advantages over conventional drying methods, such as open sun drying and thin-layer drying, in terms of energy efficiency and nutrient retention [¹⁹]. The increased thermal efficiency (11 ± 0.2%) and reduced drying time (40 ± 2.1%) observed with the proposed system suggest that the integration of paraffin wax as a thermal storage material helps maintain consistent temperatures throughout the drying process [¹⁷]. This is consistent with findings from other studies, which have shown that thermal storage materials improve the performance of solar dryers by reducing fluctuations in temperature and enhancing drying efficiency [¹⁴]. Additionally, the higher retention of total sugars, fiber, and phenolic compounds in the SDTS-dried apple slices aligns with studies emphasizing the preservation of nutrients through controlled drying conditions [¹²]. These findings suggest that the proposed dryer offers a promising, energy-efficient solution for small-scale fruit drying [³⁵]. In Figure 10, Variations in TPC, TFC, and antioxidant activity were analyzed based on the drying methods employed

Figure 10
Variations in TPC, TFC, and antioxidant activity were analyzed based on the drying methods employed.

Moreover, the cooking ratios (CR) indicated that samples dried using TD retained their structural integrity and hydration capacity better than those dried by OSD, reaffirming the results obtained by SINGH and KUMAR [³], who noted that the choice of drying method significantly affects the texture and quality of rehydrated food products. The overall analysis underscores the importance of selecting appropriate drying techniques to optimize the nutritional quality and culinary characteristics of dried apple slices, demonstrating the effectiveness of enhanced solar drying with thermal storage in achieving desirable outcomes. The apple slices used in this study were carefully prepared to ensure uniformity and optimal drying conditions. Fresh apples of uniform size and maturity were selected, washed thoroughly, and manually sliced to a thickness of 5 mm using a standardized cutting tool. The slices were then blanched in hot water (85°C) for 2 minutes to deactivate enzymes responsible for browning and to enhance drying efficiency. Post-blanching, the slices were drained and placed in a single layer within the drying chamber to avoid overlapping and ensure uniform air circulation. The thermal storage material maintained consistent temperatures throughout the drying process, effectively reducing moisture content while preserving the quality of the slices. This preparation method ensured consistent results and high product quality.

5. CONCLUSION

The study evaluated the performance of an enhanced solar dryer integrated with thermal energy storage for drying apple slices. The experimental results demonstrated that the solar dryer with thermal storage (SDTS) exhibited superior performance compared to traditional methods, achieving remarkable drying efficiency. Specifically, the moisture content of apple slices dried using SDTS reached 9.45% (w.b.), which is significantly lower than the moisture content observed in open sun drying (OSD) at 12.85% and thin layer drying (TD) at 10.20%. The drying kinetics analysis revealed that the Enhanced Page Model provided the best fit for the drying data across all methods, with high coefficients of determination (R²). For SDTS, R² was recorded at 0.9987, with a root mean square error (RMSE) of 0.0800, indicating a high level of accuracy in predicting drying behaviour.

Furthermore, the proximate composition of dried apple slices showed promising results, with protein content of 9.45% ± 0.76%, fat content of 8.55% ± 0.30%, ash content of 0.80% ± 0.04%, and carbohydrates at 69.60% ± 5.85%, reflecting the nutritional quality retained during the drying process. The rehydration ratios (RR) indicated that the SDTS method yielded an RR of 0.67, while thin layer drying achieved a higher ratio of 0.745, demonstrating that the choice of drying method influences rehydration potential. Additionally, antioxidant activities, TPC, and TFC were positively correlated with the drying techniques, emphasizing the nutritional benefits of using SDTS. These bioactive components remained higher in SDTS-dried slices compared to those processed via OSD, enhancing the health benefits of the dried apples.

The findings indicate that the integration of thermal energy storing in solar ventilation not only enhances energy utilization but also preserves the quality of apple slices, making it a viable option for maintainable drying practices in the food processing industry. This study delivers understandings into the potential for optimizing drying systems with energy storage, paving the way for further research into scaling up these technologies for commercial use. Future studies could explore the application of SDTS for other fruits, vegetables, and herbs, potentially improving food preservation in various agricultural sectors.

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Publication Dates

• Publication in this collection
  24 Feb 2025
• Date of issue
  2025

History

• Received
  28 Oct 2024
• Accepted
  12 Dec 2024