Dear Lise,
Here are the 5 principles for building a solar dryer:
130 degrees F or less
no bugs on food
as fast a wind touching food as possible
exit for all saturated air
no sun light on food
If you are in a sunny hot area there is a very simple plan that should work.
Build a six foot high four foot wide building that has sheet metal sides
painted black. Either orient the building south to north if in a very hot
sunny area or east to west if in a cooler climate and insulate the northern
side... The building should be airtight and rise up hill. It can be as long
as needed to hold the produce on screen trays inside the building. I suggest
that trays be four foot wide by two feet with a six inch gap between
screens. Screens slide onto supports nailed to the walls. Access to the
screens is through the building by full sized door located at the bottom of
the hill at the buildings entrance. A tall sheet metal chimney four feet by
four feet in diameter rises from the end of the building supported by four
wooden poles five to six inch in diameter cemented or tamped with gravel
into holes six feet deep.. The chimney is 30 feet high with a rain cap over
the top that does not interfere with the draft. The uphill end of the
building has a two foot by two foot opening in the end that allows air to
rush into the chimney. A hole that same size in the bottom end of the
building, in the door, allows air to rush in.
The black sheet metal building should be about 130 degrees inside when the
sun shines. Bug screen on openings keeps bugs out of the dryer. No sun light
contacts the food. The tall large chimney creates a wind inside the airtight
building pulling air into the building through the 2' by 2' hole in the
entrance. Saturated air exits from a 2' by 2' opening and up the chimney.
The students must tune this machine for your area. If the building is too
warm let in more air or paint it partly white or silver. If it is too cool
insulate where the sun does not contact the building. If the chimney does
not pull enough air through the dryer (if there is condensation inside) open
up the entrance and exit air holes or add a electric fan,etc. Tuning the
dryer is the fun part...try to get even temperatures everywhere in the
building. The article on the wood fired dryer has three strategies for
evening temps.
This is an idea....make a small model first to see if a larger dryer will
work...
And send us results please of this experiment. We want to help farmers dry
coffee in Central America...
All Best,
Dean
-----Original Message-----
From: Lise Goddard <lgoddard@midland-school.org>
To: dstill@epud.net <dstill@epud.net>
Date: Thursday, August 28, 2003 3:47 PM
Subject: solar dehydrator plans?
>Hi Dean,
>
>I spoke to a woman at the Aprovecho Research Center who suggested I contact
you by email. I'd like to know if you have any written plans for a solar
dehydrator comparable to the ones for the wood-fired one. I teach at
Midland School in Los Olivos, CA (southern Calif.), a private college
preparatory boarding school. The school is extremely
environmentally-oriented and this year we are redesigning the curriculum to
formalize a program in environmental stewardship that runs throughout all
the courses. Midland is on almost 3000 acres and students maintain an 11
acre organic garden.
>
>Some of us here think it would be a fantastic senior thesis/project to
build a large food dehydrator for excess garden crops and fruit trees.
These dehydrated foods could then be used as provisions on backpacking trips
that students will take throughout the year.
>
>Because we are in S. California, the wood-burning dehydrator might be
overkill, as the Santa Ynez valley is often hot and dry, except in the
winter. I'd love to get your input on this.
>
>Thanks for your time,
>Lise Goddard
>
>PS - check out Midland's website
>http://www.midland-school.org/
>