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Part VIII: Wind, Water, and BiofuelsChapter 13. Biofuels, Wood, and Solar Cooking

Solar Food and Lumber Drying

Ventilated drying boxes that use sun and airflow without cooking the load.

Difficulty
Intermediate
Cost
$40–150
published

Drying is airflow and a moderate temperature. Cooking is a closed box that gets hot. The same sunshine will do either, and the vent is what chooses. Food and lumber both need dry air moving past them. They do not share a cabinet. Sap, stain, and treated wood do not belong in anything that will later hold fruit.

This page does not set food dryness or food temperatures. The National Center for Home Food Preservation publishes the methods: nchfp.uga.edu. Follow the page for the food you have, including any pre-cook step for meat. This guide only builds a ventilated cabinet and a stickered lumber stack. Project write-ups in the links stay on those sites.

Difficulty

Level
Intermediate
Time
A weekend for a small cabinet, and weeks for a lumber stack
Skills
Keeping food out of the temperature that cooks it, and out of a sealed jar

Cost estimate

$40–150

Screen, a simple wood or cardboard cabinet, and a dark absorber panel can fit the range. Food-grade screen and a thermometer may be most of the money. A lumber stack is stickers, a cover, and space, and it can be nearly free until you buy a moisture meter. A powered fan is optional and extra.

Materials

ItemQtyNotes
A cabinet with a low inlet and a high outlet1Both screened. Air has to leave. A sealed box is the solar cooker, which is the wrong machine.
Food-safe shelvesA fewClean screen or stainless that has not held paint or pesticide. Not the rack from the shop.
A separate lumber stackIf you are drying boardsStickers aligned, cover on top, sides open, off the ground. No food in this stack.

Food: heat the air, move it

An indirect cabinet heats an absorber in the sun and lets that warm air rise through food that stays out of the direct blast. Inlet low, outlet high, both bug-screened. Shelves loose enough that air can pass. A thermometer in the air stream near the food tells you if you have built an oven. If the cabinet climbs into cooking temperatures, open the vent or shade the absorber. The official page for that food sets the range you are aiming at. A vague warm box is not "close enough" for meat, and it is a mold factory for fruit if the air is not actually moving.

Dryness is a test from that official page, not a guess that the pieces feel leathery. Conditioning, the step where dried food sits in a jar you do not seal yet so moisture can equalize, is part of their method. Do it. Then store so the food cannot reabsorb a humid week.

Do not dry food and then seal it warm and moist in a jar. Sealing moist food that way can grow botulism. Drying is not canning. Low-acid food is the strict case. If you are not sure the food is actually dry, it is not shelf stable.

Meat and jerky follow the official drying method, which commonly includes a cooking step this page will not rewrite. Do not invent a temperature for meat. Do not dry meat in a cabinet that is only "pretty warm" for a whole day. Use the food center's process or use a refrigerator and a stove.

A food cabinet and a lumber stackWarm air enters low and leaves high through screened food shelves. A separate stickered lumber stack sits beside it.Air in low, out highLumber stickersFood and lumber do not share a box.Vent the cabinet, or it cooks instead of drying.
A vented food cabinet beside a stickered lumber stack. They are different loads and they stay apart.

Lumber: stickers, cover, time

Boards dry by air moving across their faces, slowly enough not to case-harden into a ruined shell if you can help it, and fast enough not to mold. Stickers, the thin dry strips between layers, line up vertically so the weight goes straight down. Space them often enough that the boards do not sag, on the order of every foot or two for ordinary stock, closer for thin or heavy pieces. A roof or a sheet keeps rain and sun off the top. The sides stay open. The stack is off the ground on bunks that will not rot away in a month. Weight on top reduces warp.

End checks start because ends dry first. A coating on the ends, sold for that purpose, slows that. It does not replace stickers. Treated lumber, painted lumber, and anything with glue-line chemicals do not go in the food cabinet later, and they do not make a great "solar kiln" that vents into a room you breathe. A fan-assisted kiln is a box with measured temperature and a way for moisture to leave. It is not an unvented heater blowing on a pile. Do not put a combustion stove inside a kiln you built from this paragraph.

Wood for a stove still wants to get under about 20 percent moisture, which is the burning guide. Firewood rounds and furniture boards are both happier with air and a cover than with a tarp sealed to the ground.

Composite: one box, two jobs

Invented, not a real shed. Fruit goes on the racks on Monday. Tuesday the same cabinet holds a freshly stained board "just to see." Wednesday the fruit tastes like the shop. The composite ends there, before anyone eats it. Keep two piles under two shelters, and use a cabinet you can wash.

  1. Build a cabinet that must vent

    Inlet low, outlet high, both screened. Thermometer in the air by the food. If it cooks, it is not drying.

  2. Follow the food-preservation page for that food

    Especially meat. Dry is their test. Do not jar moist food and call it done.

  3. Sticker lumber in its own stack

    Cover the top, open the sides, align the stickers, keep it off the soil. No food, no treated boards in the food box.

  4. Give it time

    Fruit is days when the air is dry and moving. Lumber is weeks or months. A tarp on the ground is neither method.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.