Tuesday, January 11, 2011

Solar Fruit Dryer

The current process by which food is provided to most folks in the United States is one that uses large amounts of petroleum, from the farm through the distribution network to the home. As petroleum and other sources of energy get more scarce, we will need to find new ways - and return to old ways - that are less energy-intensive. These changes will be required through all the stages of growing, preserving, and distributing food.

Here is a simple design for a solar fruit dryer. The lower part is a trapezoidal solar collector. The top surface of the collector is clear glass. This covers a space for air to flow, in from the bottom, up through the collector as it is heated, then into the drying chamber above the collector. The bottom surface of this air space is a metal surface painted black, to absorb sunlight and turn it into heat. Below the metal surface is a layer of insulation, so the heat from the metal surface goes into the flowing air above it rather than the outside air below it.

The heated air rises and flows from the solar collector into the drying chamber. This air passes over several trays of drying fruit. The air is dryest as it enters the chamber, so the first tray it encounters is that with the dryest fruit, i.e. the fruit closest to the end of drying process. As the air rises, it passes over successively less dry fruit. Finally the air rise out through the top of the drying chamber and into the chimney.

The chimney is simply a vertical tube that helps the rising air accumulate lifting power to keep the steady movement of air in at the bottom of the solar collector, through the dryer, and out at the top of the chimney.

The drying chamber has a door allowing new trays of fresh fruit to be added at the top and trays of dried fruit to be removed from the bottom. As trays are removed from the bottom, the remaining trays should be shuffled down, creating space at the top for the fresh trays to be added.

The next stage of development of this idea will be to tune the relative sizes of the components, to allow maximum throughput of fruit with the least expense. A moderately sized unit ought to be inexpensive enough to build that this tuning can effectively be done experimentally.

The grand vision is that the dried fruit can be taken by bicycle to local farmstands to be sold, and then fresh fruit to be dried can be brought from the farmstand on the return bike trip. Mix in canoes as needed!

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