Technology
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The sun has been used to dry crops for centuries. The simplest form of solar crop drying is to let crops dry where they are grown. Solar crop drying research and development has focused on reducing the use of wood and fossil fuels for crop drying in developing countries. Some demonstration projects are also in development in North America, including manure, wool and grain drying in New York State. In addition to reducing the dependency on fossil fuels and firewood, solar crop drying research aims to reduce contamination, improve crop quality, and increase control over temperature and humidity. Demonstration projects have been implemented around the world in a variety of applications including sugarcane, coffee, nuts, herbs, rice.
One type of drying technology is solar food dryers. Solar food dryers work similarly to solar cookers, using a box with a glazed cover. To achieve the lower temperatures desired for food drying (below about 40 degrees Celsius) and drying effect, they are designed with greater ventilation and air flow, usually with several mesh racks stacked for layered drying, much like conventional electric dehydrators. Either convection or a fan is used to move the air up through the drying racks. Solar food dryers work even in humid climates, can be built for individual use or commercial scale, from readily available materials and fairly simple designs. Solar food dryers can reduce the use of electricity by replacing conventional food dehydrators.
Lumber drying accounts for a significant amount of energy use and cost in the lumber industries. Many of the kilns in use today are aging and very inefficient. Solar lumber drying represents a considerable opportunity for the lumber and solar technologies. Currently there are a few solar kilns in use in Canada with capacities of 25,000 board feet, however they can be built to any size.
Solar kilns have been demonstrated in Canada to be effective 10 months of the year, significantly reducing the need for conventional fuels. Solar kiln designs vary and use a variety of solar air heating technologies, including perforated-plate collectors and glazed collectors. Payback can be as early as the first year.
Transpired or perforated solar collectors are used in a number of solar crop research projects around the world. Similar to applications for solar air heating, transpired solar collectors are mounted on the side (or sometimes roof) of a barn or drying facility. The collectors can be connected to existing dryer fans. Perforated solar collectors, like the Solarwall, are used to heat outdoor fresh air to increase the air's capacity to absorb moisture. In addition to crop drying, solar drying can be used for manure and textile drying. The two and a half year project concentrated on displacing fuel-fired dryers for crops like tea, coffee, fruit, spices, rice, and nuts that are dried at temperatures less than 50ºC. The rate of air transfer from the solar collectors to the fans can be adjusted to allow for higher or lower temperatures, typically up to about 50 degrees Celsius. Research is also underway to explore the use of air heating via solar hot water systems in crop drying, for applications such as tobacco where greater control is required.
Canada participates in the International Energy Association's solar drying project. The project began in 2000 and involved several demonstration solar drying projects to help expand the experience and knowledge of solar crop drying applications. The potential amount of energy that could be displaced using solar in this market has been estimated to be between 300 PJ and 900 PJ annually, primarily in displacing fuel-fired dryers for crops that are dried at temperatures less than 50C. (http://www.iea-shc.org/task29/index.html TASK 29 - SOLAR CROP DRYING)
Modified: 02-13-2008
