Grantee Research Project Results
Final Report: Higher Performance Solar Crop Dryer Kit for Developing Economies
EPA Grant Number: SU833509Title: Higher Performance Solar Crop Dryer Kit for Developing Economies
Investigators: Tatara, Robert A.
Institution: Northern Illinois University
EPA Project Officer: Page, Angela
Phase: I
Project Period: September 1, 2007 through September 2, 2008
Project Amount: $9,933
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2007) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Sustainable and Healthy Communities , P3 Awards , Sustainable and Healthy Communities
Objective:
For nations with developing economies, there exists a need for a higher performing, concentrating crop dryer; such a unit must also be independent of utilities and other such energy sources. Other features must include minimal mechanical complexity and ease of operation. Replacement materials should be readily available locally. Furthermore, such a dryer ought to be available in kit form; this would provide a standardized design and be easily distributed throughout the world. Thus a potential solution is to augment the established technique of open ground drying. It is also known that the surface treatment below the drying crop can affect performance; olive-green tarpaulin, black polyethylene, and concrete surfaces have been shown to reduce drying times.
Taking advantage of existing techniques and introducing new technology, the proposed higher performing system combines concentration of sunlight, crop support surface treatment, and crop protective film covering, while keeping costs low. Accordingly, an advanced type of solar crop dryer, based on the compound parabolic concentrator (CPC), has been constructed. This unit features support brackets contoured to the calculated CPC limb profile as well as containing a reflective film. Ideally, the film would be provided in roll form. (In the installation, the film is unrolled and fastened to the brackets.) At the absorber area of the collector, a shallow housing would be located; the housing may be blackened for better solar absorption. This would hold any crop or crop in trays, connect the two sides of the support brackets together, and allow for the correct initial, or occasional, tilt adjustment of the entire collector. A fan(s), powered by a solar cell, is fixed at one end. The fan(s) create a minimal air flow to help dissipate the humidity in the collector from the moisture driven from the crop. To reduce heat losses and to shelter the crop from environmental effects, a transparent film (ideally provided in roll form) would be stretched across the aperture and fastened to the support brackets. The ends would be partly open to facilitate air circulation and to allow for crop loading. It is hoped that drying times would be reduced by at least a factor of 3. Figure 1 is a schematic view of the CPC system.
Figure 1. Cross-Sectional View of CPC Solar Crop Dryer Concept
Summary/Accomplishments (Outputs/Outcomes):
The initial design was drawn up as a preliminary draft using Pro-Engineer software. A 4X optical CPC concentration ratio was chosen, then truncated to 3X. With a receiver (crop) width of 0.5 m, the CPC aperture width is 1.5 m. This design was used as a basis for purchasing materials and hardware before actual fabrication of the CPC was undertaken. Chosen were materials that were readily available because the Phase I objective was to obtain some preliminary data showing whether or not the concept is feasible.
The first portions of the crop dryer were the supportive struts designed to hold the mirrored limbs in place above the crop trays and to elevate the trays above the ground for easy accessibility. The limbs for the reflecting surface need to conform closely to the required curvature to maintain collector efficiency. Because of this, the calculated curvature was cut on a polycarbonate plastic stencil using a computer-controlled laser at Northern Illinois University.
The struts, from 4’ by 8’ sheets of medium density fiberboard, were cut with a scroll saw and then all six were stacked for sanding. Sanding the edges not only gave the surfaces a smooth finish but it also ensured that the parts were identical. The struts were assembled into sidewalls for the collector with 16 bolts per strut and two support plates which are 14 gage sheet metal and cut by computer-controlled water jet. Lastly, the six strut halves were assembled using forty-eight 0.375’’ x 1.25’’ bolts and hex nuts as well as 96 flat washers and 48 lock washers. Figure 2 is a photograph showing the struts, braces, and cross-members holding the CPC halves together.
Figure 2. Photographic View of CPC Support Structure
Cross-members were further reinforced with wooden boards cut to approximately 8’ long with three notches each; the notches were 2” deep and spaced 3.3’ apart. Notches were cut into the struts so that the cross-members would fit into the struts. Additional cross-members ran between strut pairs and were screwed using 1.5’’ long wood screws. The base stand is 4’ by 8’ medium density fiberboard and elevated off the ground by 2” by 4” boards which also allowed for tilt motion to the entire facility. To tilt the crop dryer, eight hooks (four on the stand and four on the cross-members) were affixed and linked by an adjustable length of rope.
Two trays were designed for the bottom of the CPC; they are plywood and lined with black, waterproof plastic. The rectangular trays measure 1.0 meters by 0.5 meters by 76.2 mm deep and can hold approximately 20 kg of wet grain. To provide a track guide for the trays, angle iron, 2 meters long, was screwed and glued to the struts. Finally, the sidewalls, with the exact CPC shape, were fabricated from two pieces of smooth panel board measuring 2 m by 1.44 m and 2 mm thick. The flexible sidewall was easily snapped into the struts, then glued and screwed into place. A mirrored film will be attached to the sidewall contours and a transparent plastic plate will seal the top of the CPC to complete the solar system. Figure 3 displays the CPC structure with sidewalls, without mirrored surface, and crop trays.
The effects of air ventilation will also be tested using an array of ten small brushless DC fans powered by a 12 VDC, 1 amp, photovoltaic solar cell. A battery is used as a buffer between the solar cell and the fans to protect the fans from over-voltage and to maintain power during cloud cover. Figure 4 illustrates the fan array. Each fan has an average air flow rate of 35 cfm for a total of 350 cfm. A control box has the ability to power the fans individually to vary air cfm. The fans are mounted in a sheet of clear acrylic plastic cut by a water jet.
Figure 3. Photographic View of CPC Support Structure with Crop Trays
Figure 4. Schematic of CPC System with Fan Array
Conclusions:
The feasibility of fabricating a solar crop dryer prototype has been demonstrated. In April, the CPC unit will be set in an open field with a clear view of the southern sky. This will ensure maximum solar exposure. A grain will be soaked to a moisture content of about 20%. Solar intensity, weight of crop, and percent moisture content will be measured with time. Other recorded factors are wind velocity, outside temperature, and humidity.
Although the present design is suitable for testing, it is too bulky to be in a compact, easily shipped and assembled kit form. One recommendation would be to use plastic parts. Also, the supportive struts that hold the sidewalls and crop trays ought to be one piece.
Proposed Phase II Objectives and Strategies:
The CPC system construction is complete from Phase I. Phase II will be needed to refine data and to examine parameters of drying rates, solar intensity, circulating air, etc. Also, the ultimate crop dryer kit must be made from film-like materials for the sidewalls and top cover. Additionally, the supporting structure must be lightweight so a complete redesign is called for. The kit will be minimal in weight but with enough structural integrity to secure reflector films, crop trays, and transparent cover. Thus Phase II will involve thermal performance optimization of the CPC system and redesign for kit packaging the unit. The information to be gathered in this proposed study can be applied directly to the construction of a novel type of solar grain/crop dryer. The collector should prove more efficient than existing technology due to its concentrating capability. The kit construction will lend itself to world-wide applications, targeting developing economies of the Third World. The performance data generated will be optimized for operation.
All components would be contained in a kit, easily shipped to any part of the world. The kit would be packaged in a rectangular container of moderate size so that it could be handled by one person. The components would be relatively lightweight and installation instructions would include the tilt setting for a particular geographic location. The kit should be essentially modular so that the collector may be built to any length. Enough material would be provided for some flexibility in the length. The absorber housing would be modular, perhaps in 1 meter sections that would connect together for the desired dryer length. The crop would be loaded in from the ends.
This project enhances P3 sustainability by offering an improved methodology for drying crops. This novel, proposed solar system improves solar radiation collection at the crop; has a covering film to reduce thermal losses as well as minimize bird and insect attacks, dust, and wind dispersion; and includes concentrating limbs to increase collector efficiency, temperature, and decrease drying times. Thus the system uses proven technology while simultaneously minimizing mechanical complexity. Due to the familiarity of open sun drying, use of crop trays, etc., the system’s features would be readily accepted by the users. The limited mechanical components keep costs low, make installation easy, and part replacement locally feasible. In all, the proposed system satisfies all the requirements for use in developing nations.
Supplemental Keywords:
agriculture, crop drying, solar energy, compound parabolic concentrators,Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.