Elsevier

Renewable Energy

Volume 30, Issue 5, April 2005, Pages 729-742
Renewable Energy

Optimisation of solar tunnel drier for drying of chilli without color loss

https://doi.org/10.1016/j.renene.2004.01.005Get rights and content

Abstract

A solar tunnel drier is optimised for drying of chilli in Bangladesh. The simulation model was combined with the economic model of the solar tunnel drier and adaptive pattern search was used to find the optimum dimensions of the collector and the drying unit. Two optimum designs are obtained. For design-1, both collector and drying unit are 14.0 m long and 1.9 m wide and for design-2, both collector and drying unit are 13.0 m long and 2.0 m wide. Both the collector and drying unit of basic mode drier are 10.0 m long and 1.8 m wide. The capacity of optimum mode driers is higher than the basic mode drier and achieves a cost saving of 15.9%. The pay back period of the basic mode drier is 4 years and optimum mode drier is about 3 years. Sensitivity analysis showed that the design geometry is sensitive to costs of major construction materials of the collector and air temperature in the drier.

Introduction

Chilli is an important spice and a potential cash crop in the world. It is dried to make chilli powder and to store for both short and long term storage. In Bangladesh, a large quantity of chilli is lost during the production season when the supply is abundant. Farmers do not get a proper return for their harvest during the peak period of harvest due to the low market price. There is an increasing interest in quality dried chilli for both the local market and foreign market. In Bangladesh, chillies are traditionally sun dried. In this method, drying rate is slow, cannot be controlled and a low quality dried product is obtained. Solar crop drying is environmentally friendly and economically viable in developing countries and a quality dried product can be produced using a solar drier [1], [2], [3].

Solar drying systems must be properly designed in order to meet particular drying requirements of specific products and to give satisfactory performance with respect to energy requirements. Designers should investigate the basic parameters namely dimensions, temperature, relative humidity, airflow rate and the characteristics of products to be dried. However, full scale experiments for different products, drying seasons, and system configurations are some times costly and not possible. The development of a simulation model is a valuable tool for predicting of the performance of solar drying systems. Again, simulation of solar drying is essential to optimise the dimensions of solar drying systems and the optimisation technique can be used for optimal design of solar drying systems [1], [2], [3].

Natural convection solar drier is low cost, can be locally constructed and does not require any power and energy from electrical grid or fossil fuels. But the natural convection solar driers suffer from limitations due to extremely low buoyancy induced airflow inside the driers [4], [5]. The high weather dependent risk and drying limitations due to extremely low buoyancy induced airflow of natural convection solar driers stimulated Mühlbauer and his associates at the Institute of Agricultural Engineering in the Tropics and Subtropics, University of Hohenheim to develop a solar tunnel drier in which a fan is providing the air flow required to remove the evaporated moisture. One photovoltaic module is required to operate the fan independent of electric grid. Numerous tests in regions of different climatic conditions have shown that fruits, vegetables, cereals, grain, legumes, oil seeds, spices and even fish and meat can be dried properly in the solar tunnel drier [6], [7], [8], [9].

Several studies have been reported on optimisation of forced convection solar driers of different system configurations [10], [11], [12], [13]. Investigators [8], [9] conducted experiments for solar drying of fruits, vegetables, spices and fish using solar tunnel drier in Bangladesh. They suggested that the photovoltaic operated solar tunnel drier must be optimised for efficient operation. No study has been reported on optimisation of solar tunnel drier. The objective of this study is to optimise the physical component of the solar tunnel drier for the drying of chilli efficiently and economically without loss of its color.

Section snippets

Simulation model

A solar tunnel drier consists of a plastic-covered flat plate solar collector and a drying tunnel, as shown in Fig. 1. The drier is arranged to supply hot air to the drying tunnel using two small fans powered by a photovoltaic module. The heated air passes over and under the products spread in a single layer in the drying chamber and thus moisture is evaporated and carried away from the products.

Economic model

For calculation of the cost of drying, a simple annual cost model was used [5]. The total cost of the solar drier consists of the cost of the collector, drying tunnel, fan and PV module. The materials used in collector and drier were GI sheet, timber, glass wool, MS rod, angle bar, polyethylene cover, rubber rope, aluminium U-channel, DC fan, PV module, GI pipe, plastic net and miscellaneous materials (screw, rivet, paint, etc.). The price of construction materials in Bangladesh in the year

Optimisation procedure

In carrying out the simulations, the system was assumed to be operated from 9:00 a.m. to 4:00 p.m. daily. The economic model was combined with the simulation model. For different combinations of drier geometry (length and width) under constraint conditions of colour as well as drying air temperature (Table 3), the dimensions of the solar tunnel drier for minimum cost per unit moisture removal were determined. The optimum dimensions were determined by adaptive pattern search technique. The

Economic analysis

The economic analysis proposed for analysing the system is the annual cost analysis method. According to this method, the material cost, fabrication cost, depreciation, maintenance cost and operating cost (labor and blanching cost) are estimated and then annualized. The annual costs obtained from the estimation are employed to calculate the drying cost per unit moisture removal under the constraint conditions. The results obtained from the analysis give the optimum collector area as shown in

Conclusions

The main feature of the optimum design is a relatively long collector. Two types of optimum designs are obtained. For design-1, both collector and drying unit are 14.0 m long and 1.9 m wide and for design-2, both collector and drying unit are 13.0 m long and 2.0 m wide. Both the collector and drying unit of basic mode drier are 10.0 m long and 1.8 m wide. There is no significant difference of efficiency between the basic mode and optimum mode driers. The capacity of optimum mode driers is higher than

Acknowledgements

This piece of research has been conducted by the financial support of Commonwealth Scholarship Commission of the United Kingdom.

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