Jim'll and Tanya Build a Solar Dryer

Tanya and I are lucky enough to have a large allotment, which means that we often get large volumes of produce that need to be processed and preserved. Much gets frozen and some gets pickled or turned into jams or chutneys. However last year we fancied trying a different method for preserving crops: drying them. Now this can be done using either a normal kitchen oven on a very low setting for several hours, or in custom-made electrically powered dryers. However, both of these consume energy and, in the case of the latter, it was yet another consumer product to buy.

So we decided to try something a bit different: build ourselves a solar dryer. I'd read about these on the Internet for several years and there's already a Big Green Idea ideas sheet demonstrating the construction. We also wanted to see if it was possible to construct the dryer with what we had lying around or could acquire for free so that not only was the energy source for the drying free but so also was the equipment itself.

The first thing to do was to gather the basic components together. Most of the structure is made of wood, and that wasn't a problem as we'd already got a large stock of planks and sheets that I'd acquired from pallets, old furniture, Freecycle and other similar sources. The solar collector requires a large glass pane and we were lucky to have just collected a number of horticultural glass panes for cloches/greenhouse glazing from an ex-nurseryman who was clearing a storeroom and Freecycling lots of useful items. We actually had several panes so we selected three equally sized ones that could be stacked on top of each other to give a really thick layer of glass in order to retain heat within the collector tray. Hinges and fixings (nails and screws) were from our stock of useful bits-and-bobs in the shed (most of which we've recycled from elsewhere!) and we had two shelves from an old, broken fridge that we'd rescued ‘in case they came in useful' which would provide a hygienic, easily cleanable location for the items being dried.

Now when you're working with found and reused/recycled items you can't actually follow (or provide) a detailed plan with accurate measurements. Much of the construction is a case of working with what you've got to hand. In this case, the dimensions of width and depth of the dryer were determined by the fridge shelves, so we built a box from pallet wood and exterior grade plywood that the shelves would fit into and that gave a reasonable amount of room between the shelves and the top of the box for items to sit on. The box was arranged so that the shelves could be easily removed via a door at the front to allow easy loading and cleaning.

The next major constraint on the construction was the size of the glass panes. We could have cut the glass to a particular size but this seemed like unnecessary work and fraught with the danger of broken glass. So we constructed the solar collector around the glass panes, again using wood sourced from shipping pallets. The finishing touches were then a little stand for the box to sit on (made from the legs of an old desk) and a coat of black paint over the whole structure to help weatherproof it and also possibly help warm it up. The paint wasn't an eco-paint but it was intercepted on its way to landfill from a friend, so that didn't cost anything either. The holes at the base of the collector, between the collector and the base of the box, and at the top of the box were all covered with muslin (which Tanya had already used for jam making - we just washed it, stapled it to the wood and then let it dry to stretch it out).

The whole construction probably took a day or two in total, but it was spread over lots of smaller sessions over a month or so. By the time the dryer was ready we were approaching summer and we were looking forward to getting it into use drying lots of produce from the plot. Unfortunately we soon discovered the major failing of solar dryers: they require sun. Last summer wasn't the sunniest on record and we often missed the days that were sunny (at work, visiting friends, etc). When we did try to dry apple slices the sun just wasn't up to heating the unit sufficiently to dry the apples before mould set in. Not good.

However we did manage to get some use out of the unit towards the end of the season despite the weather. We dried out our quinoa crop and several batches of beans. The latter were both for food storage over winter and also seeds for sowing next year - the latter in for less time than the former. Damp sweet pea pods were also dried in the unit - this has definitely worked as we've just planted out the resulting plants this spring!

So how do we feel the experiment went? We certainly managed to build a dryer for free using recycled parts, so that was a positive tick in the box. The actual drying performance wasn't as good as we'd hoped, but that was mostly down to the weather and our inability to utilise what sun there was last summer. If you are relying on drying to store foodstuffs and you can't rely on periods of uninterrupted sunshine, then solar dryers might not be a great choice. Even so we did find it useful for drying the seed pods, beans and grains that came in damp from the plot.

There are some things we'll probably do different for the ‘Mark II' based on experience with the current unit. The first of these is to put a metal sheet painted matt black at the back of the collector section in order to improve the heat transfer to the air flowing up through the collector. The fridge shelves also had rather wide slats - we had to put most items on paper sheets or muslin to stop them falling through the gaps. This of course affects the all-important air flow around the items being dried - ideally we'd like to get some fine mesh or gauze to lay items on. Finally, whilst we made the unit in three parts (the collector, the shelving box and the stand) that could be quickly dismantled for transport, it still takes up quite a bit of room. It would be good if we could devise some way for the unit to all pack inside the main box for transport and storage.

Still, not bad for a few hours in the shed and some items saved from landfill! Let's just hope we get some more sun this summer!

Jim'll

photos (c) Jim'll and Tanya 2009

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