Evaluation of sectrally-selective materials for multi-layer solar thermal crop drying (abstract)

Authors: Alleyne, Fatima; Milczarek, Rebecca; McHugh, Tara

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/23/2015
Publication Date: 7/12/2015
Citation: Alleyne, F.S., Milczarek, R.R., Mchugh, T.H. 2015. Evaluation of sectrally-selective materials for multi-layer solar thermal crop drying (abstract). Institute of Food Technologists Annual Meeting, Chicago, IL; July 11-14,2015.

Interpretive Summary: This work summarizes a quantitative, scientific evaluation of potential material candidates for multi-layer solar thermal (ST) dryers to determine the 1) feasibility to incorporate phase change materials into our multi-layer design drying system, 2) ambient conditions for efficient drying, 3) moisture content of model material, and 4) drying rate of model system under various conditions. Although ST drying has had widespread use in developing countries for produce preservation; it has had limited commercialization in the United States due to slow drying rates and poor product quality. Most of the published articles in the literature provide insight on the performance of solar dryers in service but with little information on the selection process or material attributes that allow them to be selected as candidates in solar dryer designs. Furthermore, if solar dryers can be designed such that the drying process can occur over a 24-hour period the efficiency of drying can be improved. Thus, the most suitable unit for large scale farm drying would entail the incorporation of phase change materials (PCMs) as heat storage devices, with an intended application in fruit and vegetable crop preservation. In order to incorporate PCMs, dryer units must at least achieve the phase transition temperature of the PCM under minimal conditions. Candidate materials of interest possess phase transition temperatures of at least 40C; thus, we need to achieve at least this temperature during diurnal drying to capitalize on the heat storage mechanism of PCMs as a heat source during overnight drying. To identify the best materials suitable for solar thermal dryers a scientific investigation of spectrally-selective materials was employed to assess the internal dryer temperature with respect to ambient conditions, heat retention, product moisture content, and drying rate. Results of this work will enhance solar thermal drying efficiency, product quality and crop preservation capabilities.

Technical Abstract: Solar thermal (ST) drying is a ubiquitous method in widespread use for fruit and vegetable crop preservation in developing countries; however, it has had limited commercialization in the United States due to concerns about slow drying rates, poor product quality, and predicted low return-on-investment. If solar dryers can be designed such that the drying process can occur over a 24-hour period, via phase change materials (PCMs) or other heat storage mechanisms, the efficiency of drying can be improved. To achieve this, potential PCM candidates have been identified with phase transition temperature of 40C. In this work, we constructed and tested 4 multi-layer ST dryers to assess the effects of spectrally-selective cabinet materials on the temperature profiles, moisture content, and drying rate of sponges (model material). In this study, rectangular shaped sponges were utilized to model drying behavior. Multi-layer drying cabinets were constructed utilizing various materials: stainless steel (SS) and acrylics - extruded (Ex), ultraviolet blocking (UV), and Solar Sheet (S). Three replicate drying runs were performed during November 2014. Temperature profiles within the drying cabinets, meteorological conditions, and sponge weights were recorded during drying. Temperatures of 40C and higher were only observed in the Ex and UV dryers. Drying rates were material dependent, with lower moisture content values, at the shortest times, observed in sponges dried in the Ex dryer at 300 minutes, in comparison to >360 minutes for all others. Distinctions in drying can be attributed to differences in optical (absorbance) and thermal (thermal conductivity) properties of materials. Thus, in this work we have demonstrated that even under ambient conditions of 25C the minimum drying conditions can be achieved to design a solar thermal dryer incorporating PCMs. Results of this work will be utilized to develop the second iteration for specialty crop drying of apricots.