Comprehensive Performance Evaluation and Life Cycle Analysis of a Pebble Bed-Integrated Forced Convection Solar Dryer for Agro-Processing

Abstract

This paper presents a rigorous thermal, exergetic, environmental and economic performance evaluation of a single-glazed, single-pass, forced-convection indirect solar dryer integrated with a packed-bed sensible heat storage (SHS) unit. Experimental investigations were performed in Coimbatore, India (11.0183°N, 76.9725°E) using a 2 m² corrugated galvanised iron absorber plate tilted at 11° and a 110 kg pebble packed bed (30–50 mm diameter, porosity ? = 0.85) acting as the SHS medium. Eight air mass flow rates ranging from 0.0141 kg/s to 0.0872 kg/s were evaluated. Experimental results confirm that the SHS unit extends collector operation by 2 h post-sunset, maintaining air temperatures 6–8 °C above ambient. Bitter gourd (Momordica charantia) slices (91.75 % w.b. initial moisture) and curry leaves (Murraya koenigii, 67.3 % w.b.) were dried to final safe moisture levels. At the optimal flow rate of 0.0636 kg/s, bitter gourd drying was completed within 7 h with a maximum dryer thermal efficiency of 19.12 %, an SMER of 0.225 kg/kWh and a minimum specific energy consumption (SEC) of 4.440 kWh/kg water. Thin-layer drying kinetic analysis identified the Two-Term model (R² = 0.998646) for bitter gourd and the Modified Henderson and Pabis model (R² = 0.999337) for curry leaves as the best mathematical representations. Life cycle assessment revealed an embodied energy of 1109.3 kWh, an energy payback time (EPBT) of 2.21 years, a financial payback period of 2.71 years and a net lifetime CO? mitigation of 13.07 tonnes over a 15-year lifespan.

Introduction

The text presents the design, mathematical modelling, experimental evaluation, and sustainability assessment of an indirect forced-convection solar dryer integrated with a granite-pebble sensible heat storage (SHS) system for agricultural products such as bitter gourd and curry leaves.

Traditional open-sun drying is inexpensive but causes problems such as uncontrolled drying, contamination, insect infestation, rain damage, and product losses. Solar dryers improve hygiene and drying quality, but their major limitation is the intermittent availability of sunlight. To address this, the study integrates a 110 kg granite-pebble packed bed into the solar air collector to store thermal energy during sunny periods and release it when solar radiation decreases or after sunset.

Main objectives

The investigation aims to:

• Develop energy-balance models for solar air collectors with and without pebble storage.
• Evaluate outlet temperature, thermal efficiency, thermohydraulic efficiency, pressure drop, and exergy destruction at different airflow rates.
• Study drying kinetics, moisture diffusivity, specific energy consumption, and drying models for bitter gourd and curry leaves.
• Assess embodied energy, energy payback time, CO? reduction, and life-cycle cost.

Experimental system

The system consists of:

• A 2 m² single-pass corrugated solar air collector.
• A 110 kg granite-pebble storage bed.
• A 0.45 kW centrifugal blower.
• A flow-control section.
• A four-tray insulated drying chamber.

The system was tested at the Solar Energy Research Centre in Coimbatore, India.

Mathematical modelling

The study uses energy-balance equations to determine temperatures of the glass cover, absorber, air, pebbles, and backplate. Heat-transfer correlations are applied for different airflow regimes, while the Ergun equation is used to estimate pressure drop through the pebble bed.

The study also evaluates:

• Thermal efficiency (ηth)
• Effective thermohydraulic efficiency (ηTHE), accounting for blower power
• Pressure drop
• Exergy efficiency and exergy destruction

Key results

At an airflow rate of 0.0447 kg/s, the collector achieved its highest performance around 13:00, when solar irradiance reached 882 W/m².

At this time:

• Absorber temperature reached 68.2°C.
• Outlet air temperature reached 54.2°C.
• Thermal efficiency reached 46.86%.
• Thermohydraulic efficiency reached 46.07%.

The exergy analysis showed that the largest source of irreversibility was the temperature difference between the absorber and the sun (57%), followed by optical losses (33%). Environmental losses accounted for 6%, absorber-to-air heat-transfer losses for 3%, and only about 1% was recovered as useful output exergy.

Importantly, the pebble storage bed continued releasing heat after solar radiation declined, maintaining an outlet temperature of about 35.2°C at 18:00, which was approximately 3.1°C above ambient. This demonstrates the usefulness of sensible thermal storage for extending drying beyond periods of strong sunshine.

Conclusion

1) Incorporating a 110 kg granite pebble sensible heat storage bed extends collector operation by 2 h post-sunset, delivering air 6–8 °C above ambient and suppressing off-sun thermal collapse. 2) The optimal air mass flow rate for agricultural drying is 0.0636 kg/s. This operating point achieves a peak collector thermal efficiency of 43.10 %, a dryer thermal efficiency of 19.12 %, a minimum specific energy consumption of 4.440 kWh/kg water and an SMER of 0.225 kg/kWh. 3) Flow rates exceeding 0.0636 kg/s induce steep pressure drops (> 110 N/m²), causing parasitic blower power to drive the net collector exergy efficiency negative. 4) Thin-layer kinetic modelling established the Two-Term model (R² = 0.998646) for bitter gourd slices and the Modified Henderson and Pabis model (R² = 0.999337) for curry leaves as the optimal empirical formulations. 5) The environmental and economic analysis yields an energy payback time of 2.21 years, a financial payback period of 2.71 years and 13.07 tonnes of net lifetime CO? mitigation, confirming the thermodynamic and economic viability of the system for commercial rural implementation.

References

[1] Akpinar, E. K. (2010). Determination of suitable thin layer drying model for some vegetables and fruits. Energy Conversion and Management, 51(1), 208–216. [2] Akpinar, E. K. (2011). Mathematical modelling and exergy analysis of solar drying of green pepper. Energy Analysis of Agricultural Systems, 31(1), 20–31. [3] Alwan, Z., & Jones, P. (2014). The importance of embodied energy in carbon footprint assessment of buildings. Energy and Buildings, 68, 560–568. [4] Chowdhury, M. M. I., Bala, B. K., & Haque, M. A. (2011). Energy and exergy analysis of solar drying of jackfruit leather. Biosystems Engineering, 110(2), 108–115. [5] Dhiman, P., Thakur, N. S., & Chauhan, S. R. (2011). Thermal and thermohydraulic performance of counter and parallel flow packed bed solar air heaters. Energy, 36(5), 2712–2722. [6] Doymaz, I. (2006). Thin-layer drying behaviour of mint leaves. Journal of Food Engineering, 74(3), 370–375. [7] Doymaz, I. (2007). Air-drying characteristics of tomatoes. Journal of Food Engineering, 78(4), 1291–1297. [8] El-Sebaii, A. A., & Shalaby, S. M. (2013). Solar drying of agricultural products: A review. Renewable and Sustainable Energy Reviews, 16(1), 37–43. [9] Erbay, Z., & Icier, F. (2009). A review of thin layer drying of agricultural products. Critical Reviews in Food Science and Nutrition, 49(5), 441–456. [10] Fudholi, A., Sopian, K., Ruslan, M. H., & Alghoul, M. A. (2014). Performance and economic analysis of solar drying system. Renewable and Sustainable Energy Reviews, 33, 521–529. [11] Hedayatizadeh, M., Chaji, H., & Najafi, G. (2016). Exergetic performance optimization of a solar air collector. Solar Energy, 139, 47–58. [12] Hollands, K. G. T., & Shewen, E. C. (2001). Optimization of V-corrugated solar air heaters. Solar Energy, 26(1), 23–31. [13] Holman, J. P. (1994). Experimental Methods for Engineers (6th ed.). McGraw-Hill, New York. [14] Madamba, P. S., Driscoll, R. H., & Buckle, K. A. (1996). The thin layer drying characteristics of garlic slices. Journal of Food Engineering, 29(1), 75–97. [15] Matheswaran, M. M., Arjunan, T. V., & Senthilkumar, D. (2018). Analytical exergy evaluation of single pass jet impingement solar air heater. Journal of Thermal Analysis and Calorimetry, 132(1), 583–596. [16] Mohanraj, M., & Chandrasekar, P. (2009). Performance of a forced convection solar dryer integrated with gravel as sensible heat storage material. Solar Energy, 83(7), 1128–1133. [17] Shanmugam, V., & Natarajan, E. (2006). Experimental investigation of an indirect solar dryer integrated with desiccant bed. Renewable Energy, 31(7), 1017–1028. [18] Tiwari, S., & Tiwari, G. N. (2016). Energy, exergy, environmental and economic analysis of an active solar distillation system. Desalination, 398, 121–129. [19] Usub, T., Lertsatitthanakorn, C., & Wiset, L. (2010). Experimental performance and economic evaluation of an indirect solar tunnel dryer. Applied Energy, 87(1), 127–133. [20] Yaldiz, O., Ertekin, C., & Uzun, H. I. (2001). Mathematical modeling of thin layer solar drying of green pepper. Energy, 26(5), 457–465.

Copyright

Copyright © 2026 Vishal Chawda, Mrigendra Singh, Priyanka Malviya. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.