Build a solar dryer, and angle it for the season

Learn the tilt once, use it three ways

A solar dryer, a solar oven and a solar panel are all the same geometry problem: present the largest possible area square-on to the sun.

The arithmetic below is therefore the tilt page for the whole app. It applies to the panel in sizing a 12V system exactly as it applies to a drying rack.

Get it wrong and you lose real output. A panel lying flat in December collects a fraction of what the same panel collects tilted properly, because the sun is very low and a flat surface presents almost nothing to it.

The tilt arithmetic

Start from your latitude. In Britain that is roughly:

Two things worth noticing in that table.

British winter tilt is steep. Around 67 degrees is closer to vertical than to flat, and it looks wrong. It is not. The December sun in Manchester barely clears 13 degrees above the horizon at noon, so a near-vertical surface is the one facing it.

The gain from adjusting is modest. Five to ten per cent is worth having on a permanent installation you visit anyway, and it is not worth building a complicated adjustable mount to chase. Fixed at the latitude angle is a perfectly good answer. If you can prop it steeper for winter with a length of timber, do that, and do not build a tracker.

A wall-mounted panel is not the disaster it looks. A vertical south-facing wall is at 90 degrees, only about 23 degrees off the ideal winter angle, so it performs surprisingly well in the season you most need it and poorly in the season you least do. For a winter-biased system that is an acceptable trade, and it sheds snow and stays clean.

Doing it without a protractor

You will not have a protractor and you do not need one.

• Any phone has an inclinometer in its compass or measure app. Rest it on the frame.
• The 3-4-5 method: a right angle from three sticks, then a plumb line.
• A shadow tells you directly. Stand a stick vertically on the surface. When the stick casts no shadow, the surface is square to the sun. That is the whole of it, no measurement required, and it works for a dryer, an oven and a panel identically.
• Noon shadow gives you south. The shortest shadow of the day points due north in Britain, so the sun is due south at that moment. Mark it once on the ground and it is true forever.

The shadow method is the one to teach children, because it needs nothing, cannot be got wrong, and demonstrates the principle rather than the number.

Why a solar dryer is not a solar oven

They look similar and they do opposite things.

• A solar oven traps air to get hot. It wants a sealed box, glazing, insulation and no airflow.
• A solar dryer moves air. It wants heat and a continuous current of air passing over the food and carrying moisture away.

A sealed box full of wet food is a mould incubator, not a dryer. If you build a solar oven and put fruit in it, you will steam the fruit. Airflow is the whole design problem.

The indirect dryer, which is the one to build

Two chambers, connected.

1. A collector. A shallow, glazed, black-lined box, tilted to the sun, open at the bottom and open at the top into the drying chamber. Air enters at the bottom, heats as it passes over the black surface, and rises.
2. A drying chamber. A vertical cabinet above the collector, holding mesh trays, shaded rather than glazed, with a vent at the top.

Warm air rises through it by itself. No fan, no power. That is the elegance of the design and the reason it is worth building rather than improvising.

Why indirect rather than direct:

• Sunlight destroys vitamins, particularly vitamin C and the B group, and bleaches colour badly.
• A shaded chamber dries more evenly, because the tray nearest the glass is not cooking while the one below stays damp.
• It is easier to keep insects out of a chamber than off an open rack.

A direct dryer, where food sits under glass, is quicker and cruder. For anything you are going to store rather than eat this week, indirect is better.

Building it

The collector:

• As large as the chamber can use, and roughly the same footprint is a reasonable starting point.
• Glazed with glass, polycarbonate or clear plastic sheet. Old window units are ideal and are often free.
• Black inside. Matt black, not gloss. Black-painted metal sheet or thin black-painted board is the classic absorber. Matt matters because gloss reflects.
• Air gap of 50 to 100 mm between absorber and glazing.
• Inlet at the bottom, meshed against insects.
• Tilted per the table above, on its own legs or propped.

The chamber:

• Insulated or at least shaded. Any box will do. Timber, an old cupboard, a stack of crates.
• Mesh trays, food-safe and removable, spaced 75 to 100 mm apart. Stainless mesh, food-grade plastic mesh or muslin over a frame. Not galvanised mesh in contact with food, because zinc migrates into acidic foods.
• An adjustable vent at the top. This is your only control, and it matters: closing it down holds heat, opening it moves more air. Damp days want it open.
• A door you can open without unloading the whole thing.
• A thermometer inside. Cheap, and it turns the whole thing from guesswork into a process.

Aim for 45 to 60°C inside the chamber. That is the same range an electric dehydrator uses, and it is what drying and jerky is written around.

The British humidity problem

Sunshine is not the constraint here. Humidity is.

Air can only hold so much water. Warm air holds more than cold air, which is why heating the air is what makes drying work at all. But if the air arriving is already close to saturated, warming it a little does not give it much extra capacity, and the food dries slowly.

Slow drying is the failure mode. Food that takes four days instead of one moulds while it is drying, and you lose the batch.

What this means in practice:

• Choose your days. A dry, breezy, sunny day is worth three humid ones. Do not start a batch on a muggy overcast day because the dryer is free and available.
• Cut thinner than you think. Thickness is the single biggest variable you control, and halving it more than halves the drying time.
• Get it moving fast on day one. Most of the risk is in the first several hours, while the surface is still wet.
• Finish indoors if you must. A batch that is nearly dry at dusk should come inside, not sit in a cooling chamber overnight collecting condensation.
• Do not leave it out overnight. As the chamber cools below the dew point, moisture condenses back onto the food, which undoes the day's work and is how a batch that looked fine at 6 pm is slimy at 8 am.
• Have a backup. In a genuinely wet week, solar drying will not work at all. This is an August technique in Britain, not a September one, and pretending otherwise loses food you cannot spare.

This is why solar drying is a supplement in this climate rather than a preservation strategy. Fermenting, root cellaring and canning carry more weight here, and they work in November.

Running it

• Load in the morning, so the food gets the full day.
• Rotate the trays at least once. The bottom tray is always in the warmest, driest air.
• Do not overload. Single layer, pieces not touching. Crowding is the commonest reason a batch fails.
• Pre-treat where it helps: blanching most vegetables, and an acid dip of lemon juice or vinegar for apples and pears, which keeps colour and slows spoilage.
• Watch the thermometer. Above about 65°C the outside case-hardens, sealing moisture inside, and the food then spoils in storage looking perfectly dry.
• Test properly. Fruit should be leathery with no moisture when torn and squeezed; vegetables should be brittle.
• Condition before storing. Loose in a sealed jar for a week, shaken daily. Any condensation on the glass means it is not dry, and back it goes. This step catches the batch that would otherwise have spoiled in the store.

Improving a solar oven

The same principles, applied to the box in solar oven.

• Tilt it. Aiming a solar oven properly is the largest single gain available and costs nothing.
• Re-aim it every 30 to 45 minutes. The sun moves 15 degrees an hour, and an oven that was aimed at noon is well off by two.
• Add reflectors. Extra panels of foil or a car windscreen reflector bouncing light onto the glazing can add a great deal. Four flaps around the opening is the standard arrangement.
• Double glaze it. Two layers with an air gap between them cuts heat loss substantially, and in Britain heat loss matters more than in the climates these designs come from.
• Insulate everything but the glazing. Wool, straw, crumpled paper, anything dry.
• Black, matt, thin-walled cookware. Shiny pans reflect the energy back out of the box.
• Start early. A solar oven is slow, so lunch goes in at nine, not at eleven.