Solar Pasteurization Project
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Development of a Second-Generation Solar Pasteurizer for Soil-Based Potting Mixes.
Sheldon C. Furutani and Marcel M.C. Tsang
University of Hawaii at Hilo, College of Agriculture, Forestry and Natural Resource Management, 200 W. Kawili Street, Hilo, HI 96720-4091
Objectives:
- Design and build a second-generation solarization unit that can pasteurize soil-based media.
- Build a working proto-type of a potting media solarizing unit that could be applied to commercial nursery operations in Hawaii.
Introduction
The basic concept in our first generation solarization
unit was to pasteurize potting media with passive solar energy. In this
system, the potting media were contained in copper tubes, called solar
collectors; these collectors heat the media with radiant solar energy.
The original unit was held in a predetermined 'fixed' orientation to collect the maximum amount of incoming solar radiation. Styrofoam insulation panels were added to the interior walls of the collector to reduce radiant heat energy loss from the solar collectors. An inexpensive polyethylene film, Monsanto 602 film, 6 mils thick was selected as the glazing material for the solar collector. This glazing material is commonly used to cover greenhouse roofs that make it inexpensive and readily attainable at most garden stores. While the original unit is very effective for pasteurizing (130 to 140°F) fungal plant pathogens like Pythium from potting mixes, the unit does not take full advantage of incoming solar irradiation. In addition, there is a need to elevate media temperature above that attained by the original solarization unit. The additional higher media temperature is needed to disinfect parasitic nematodes from the recycled potting soil.
Elevated pasteurization temperatures will also be useful in eliminating certain problem weeds that commonly contaminates recycled potted media especially those containing soil. One such weed which commonly contaminates soil based potting media is the purple nutsedge, Cyperus rotundus, a weed that has earned its reputation as the 'The worlds worst weed'. The dormant tubers of the purple nutsedge are especially difficult to control even with the use of chemical herbicides. Solar pasteurization treatment would provide an excellent non-chemical alternative method of weed control. Chemical controls are considered too costly and too problem-sum to utilize since these chemicals often leave residues that hinder the production of subsequent crops.
We can easily attain higher media temperature with the original solar pasteurized by reducing the diameter of the collector tubes. This, however, would also drastically reduce the 'treating capacity' of the unit. Thus, the project objective is to improve the efficiency of the original solarization unit. We wanted to improve the efficiency of the solar unit by incorporating a solar tracking platform that would allow the unit to track the movement of the sun. By tracking the sun, the unit would collect maximum solar irradiation. Solar tracking would also eliminate much of the shadows cast by adjacent collectors, especially when the sun is low in the sky. Shadows cast by collectors tend to create 'cold spots' which results in non-uniform media pasteurization.
Materials and Methods
Construction of a fixed and a tracking solarization unit.
Two 4 ft by 4 ft. solar collectors were constructed, each were lined with 1 inch thick Styrofoam (type B) insulation. Three copper cylinders were fabricated from 3 x 10 ft flat stock sheets for each solarization unit, their dimensions were 48-inches long by 6, 9 and 12-inches diameter (See fig. 1). The flat stock sheets were fabricated into cylinders by first cutting the stock sheets to size then passing it back and forth thru a roller. The rolling creates a symmetrical cylinder. The seams were secured by crimping and by soldering edges. The cylinders were fitted into the solar collector boxes and held in place with copper brads. End caps were fabricated into 2-inch styrofoam plugs which fited tightly into the open cylinder ends. The Styrofoam plugs provide a good thermal seal (Fig. 2).
Fig. 1. Solarization units. Copper collectors installed.
Fig. 2. End view of solarization unit. Left. Copper cylinders
have been filled with media and temperature probes have been inserted.
Right. Styrofoam end-cap. Provides thermal seal for solar collectors.
While the construction of the solarization units was relatively straight forward, the development of the solar tracking mechanism was not an easy task. Many ideas led to equipment that was either to complex and/or too expensive. The selection of the tracking system literally took months with numerous hours on the telephone and internet, communicating with hydraulic engineers, mechanical engineers, step-motor specialist and numerous other people involved in electronics. The problem was the development of a mechanism that could move the solarization unit very slowly (360 degrees in 12 hrs to track the sun, this breaks down to moving the solarization unit 1 degree every 2 minutes). This slow tracking compounded with the weight of the solarization unit, approx. 1,000 to 1,500 pounds for the 4 x 4 ft unit. The closest thing we found on the market was a telescope-tracking device; it had a capacity of moving a maximum weight of 30 pounds and was advertised at seven thousand dollars. This was far too expensive and too limited in capacity for our solarization unit. We then decided to abandon the idea of continuous tracking and develop a discrete tracking system instead. A discrete tracking system does not track the sun continually, but moves incrementally. We located a device known as a linear actuator (Saginaw ball-screw actuator) (See fig. 3) that is an ideal solution to move th solar pasteurization unit.
Fig. 3. Saginaw linear accuator (ball-screw type).
Fig. 4. Uniden Supra satellite receiver.
To program the ball-screw linear actuator, we purchased a programmable satellite receiver (Uniden Supra satellite receiver). The receiver has 5 programmable settings (determines the exact position of the actuator) that provide discrete movement to 100th of an inch (See fig. 4a). The linear actuator and the satellite receiver were purchased for under $600.00.
Fig. 4a. Left. End-view of solar tracking unit.
Right. Linear actuator mounted onto frame
A sturdy welded pipe frame was then constructed (See fig. 5). The axle that held tilt the solarization unit was another problem that needed to be solved. The axle needed to rotate with the suns' movement yet be able to move up and down to adjust for the inclination changes needed for the changing seasons. The axle needed to have a more vertical orientation for winter months and more horizontal orientation for summer months. This was solved with a radial ball bearing. A bearing that rotated and swiveled at the same time. A special mount for the bearing was fabricated by a local machine shop. The mounts are known as a Pitman bearing holder (See fig. 6).
Fig. 5. Top left shows the base frame construction.
Top right displays the bottom axle assembly. Bottom left
and right illustrates the height adjustable top axle in the extended
position to accommodate season changes.
The construction of the solar units (one fixed non-tracking unit and one tracking unit) and the tracking platform were completed in late July and were moved to the area adjacent to the UHH-CAFNRM building (See fig. 6).
Fig. 6. Tracking and nontracking solarization units located
adjacent to UHH-CAFNRM.
Higher media temperature was attained with the solar tracking platform.
The tracking solarization unit collected greater quantities of solar irradiation compared to the non-tracking unit . Overall, approximately 20% more solar energy (Watts per meter squared) was collected by utilizing the solar tracking platform. In result, the solar tracking unit had as high as 5 to 7% higher media temperature compared to the non-tracking unit. The tracking platform increased the media temperature within the 6 inch collector an average of 15 °F (178 °F) compared to the non-tracking unit (168 °F). (Core media temperature recorded within the collector).
Despite the increased efficiency of the tracking platform, we felt that the solarization unit was not performing optimally for the additional solar irradiation collected. The inefficiency seemed to be linked to the solar glazing material. Solar glass glazing is well known to be a more efficient glazing then plastic and ordinary glass and is commonly used to glaze solar hot water heaters, however, their cost is very prohibitive. The solar glass glazing is manufactured with low iron materials and the glazing is also tempered. Tempered glass cannot be cut with standard glass cutting tools. We were quoted about $300 to $400 to glaze one of our 4 x 4 ft units. Reports on solar glazing report a decrease of 3 to 5 % by using ordinary plate glass (3/16 Inches thick - recommended). The cost of this type of glazing is less then 1/4 the cost of solar glazing ($80 for 4 x 4 ft with polished edges) and is readily available from local glass shops. We purchased the ordinary glass glazing and tested it against the polyethylene film glazing material.
Comparing plate glass vs. polyethylene film (greenhouse glazing) as solar glazing.
The plate glass glazing was installed on the tracking unit and the unit was held stationary position identical to that of the fixed tracking unit. The fixed tracking unit was glazed with the 6 mil polyethylene film. The comparison between glazing materials could then be observed.
On average the glass glazing material improved the media temperature approx 5% over the 6 mil polyethylene film. The 9-inch collector media temperature was increased from 132 to 138 °F and for the 12-inch collector from 121 to 128 °F with the glass compared to the polyethylene glazing material.
Experiment on pasteurizing soil-based potting media containing viable tuber of Purple nutsedge.
Soil based media containing viable tubers of Purple nutsedge were solar pasteurized in the tracking solarization unit between October 22 to 27th, 2002. The weather during this period was mostly cloudy on October 22 to 23rd; and mostly sunny between October 24 to 27th. Purple nutsedge tuber eradication occurred only in the 6-inch solar collector. The eradication temperature for tubers was above 140 °F. Unfortunately the logger in the 6-inch collector became defective during this period, but past data indicates that media temperatures probably exceeded 175 °F, and was above 150 °F for more than 8 hours during this period.
Table 1. Purple nutsedge tuber germination after various solar treatments.
| Collector size | % Germination (x) | Max.temperature °F | No. hours above 130 °F |
|---|---|---|---|
| 6 inch | 0 | Logger damaged (y) | Not available (z) |
| 9 inch | 20 | 139.8 | 8 |
| 12 inch | 50 | 112.8 | 0 |
| Control | 55 | N/A |
(X) Germination recorded after 1 month.
(Y) Logger failure. Estimated 175 °F max temperature.
(Z) Above 150 °F for several hours.
Conclusion
Glazing material and tracking platform modifications have proven to increase the media temperature within the solar collectors. The glass glazing increases media temperature about 5% and the tracking platform increase is about 7%. These are substantial increases in efficiency.
The original solarization unit we designed is excellent for temperature ranges shown in Figure 8. Maximum temperature is approx 160 °F with the 6-inch collector at the UHH-CAFNRM site. Desired temperature beyond this range would require a reduction of the collector diameter or the use of the plate glass glazing material. This modification will increase the temperature from 168 to 180 °F, an increase of 12 degrees. Still higher temperature is possible with the solar tracking platform, but the added cost of the tracking platform may be economical only in specific cases. An application that may require this added cost would be the eradication of purple nutsedge from potting media where very high temperature must be attain.
The tracking platform is a very important platform that can be easily modified for other devices that use solar energy. Hot water heaters, driers, and other types of solar collectors would definitely benefit for the use of this platform.
We would also like to thank the Hawaii County R&D for making this research possible.