October 22-24, 2014 Bogot , Colombia

Experimental comparison of the effectiveness for various collector elements

in a Hohenheim-type solar dryer Ivn Guillermo Surez; Orlando Porras Rey

Department of Mechanical Engineering. University of Los Andes ig.suarez2261@uniandes.edu.co; oporras@uniandes.edu.co

1. Introduction

The University of Los Andes has studied the solar drying of sago (Canna edulis) starch in a solar tunnel dryer adapted from the original design by the University of Hohenheim.

It is known that this solar dryer works fine. However its energy efficiency is low and we suspect it can be optimized. This work focused on the experimental study of solar collector element alternatives. Variables like collector element geometry and surface finishes can be changed to analyze the improvement of solar radiation absorption and heat transfer to the drying air.

2. Materials and Methods

Different configurations for the collector element was proposed to develop three types of alternatives: an aluminum sheet with sandblasting surface finish, a steel sheet painted with a special coating for solar collectors called Thurmalox, and a bed made of aluminum lathe chips coated with Thurmalox.

Six series of experiments were performed on two available single-glazed dryers with identical geometry running simultaneously; in one of them the collector element is changed, and the other is kept as a reference. In each experiment meteorological variables such as solar radiation, ambient temperature and humidity, and wind speed and direction were measured and registered every five minutes. On each dryer, the surface temperature of the collector element and the temperature of the heated air were measured. Also sago

October 22-24, 2014 Bogot , Colombia

starch samples were taken every twelve hours during the experiment for later moisture content measurement.

3. Results and Discussion.

Characteristic curve of the thermal behavior of each configuration of solar collector element was obtained. Drying kinetics and drying rate were determined and compared. By studying these graphs, a hysteresis phenomenon between the collector element temperature and the solar radiation was identified.

4. Conclusions

The hysteresis between the collector element temperature and solar radiation allows for better correlations between the variables involved in the drying process.

The Thurmalox coating absorbs more efficiently the solar radiation producing higher surface temperatures in the collector element.

It is demonstrated experimentally that the collector element formed by aluminum chips coated with Thurmalox is the best configuration for a greater the solar dryer efficiency.

Keywords: solar tunnel dryer, collector element material, sago, Canna edulis, solar collector coating.