Second solar food dryer finished!


About a year ago a subscription issue of Homepower Magazine arrived at our home. One of the stories was about a solar food dryer. (Issue #199, page 58) I pine for an opportunity to do proper solar in our home but for various reasons--mostly siting and financial--it hasn't been practical. The solar food dryer was a solar project I could afford and accomplish and it fit well with my interests. I share it here because I haven't seen many web photos or discussion of this particular dryer and maybe I can offer some of my own experiences with its use later during the harvest season.

Our inner Northeast Portland (Oregon) neighborhood is filled with old turn-of-the-century fruit trees. We have none in our yard but we (my daughter, Greta, and I) forage off of neglected trees in vacant and commercial lots and overhanging branches. Over the years I've put up a fair amount of fruit but canning gets to be a hassle after a while. It's time consuming and labor intensive. Canning fruit, from start to finish, can take up a weekend and sometimes it only happens if a free weekend falls when the fruit is ripe. With plums in particular you need to put them up shortly after picking them. You can't put them in a box and forget about them like you can with apples.

For the past few years we've been drying a lot of plums in a garage-sale-found electric food dehydrator. The electric dryer works well but it is noisy and it does add noticeably to our monthly electric bill, maybe $10 extra for a month.

Dried plums, aka PRUNES, are the bomb. Say "prune" and all sorts of unpleasant associations are conjured up. As a youngster, I drank the better part of a full jar of prune juice in one sitting. I can attest to the unbelievably effectiveness of prune juice as a bowel mover. It gave me hours of gut wrenching pain on the toilet.

Prunes or dried plums however are pretty much like any other dried fruit and don't deserve the same rep as prune juice. They're sweet and flavorful. They keep well and make a fantastic snack or additive to things like oatmeal and baked apple crisps. Portlanders used to understand the incredible urban agricultural value of of the free-stone Italian plum. They are all over the place here and mostly the fruit goes to waste.

When life gives you plums make pluminade!

The solar food dryer seemed like a big enough project that I would get a better return on my investment in time if I made two instead of one. The idea made sense since I only had to dig my table saw out from the garage a few times for the extra effort and I would have less waste if I bought material for two dryers instead on one. After purchasing the full book (the HP article doesn't have the complete plans), The Solar Food Dryer by Eben Fodor, I finished one dryer last year mid-summer, as a gift for a friend. I figured she needed it sooner than I did. (She has a huge garden.)

I've finally gotten around to getting number two done.

Here it is looking down on the dryer, like the camera is the sun:




Below is the rear of the dryer with the doors open and the screen drying trays partially pulled out.
A bit of construction detail
I deviated from the plans a bit by adding extra bubble foil insulation on the very bottom piece of plywood (under the spray-painted black sheet metal "collector plate." I also added standard kitchen aluminum foil on bottom front and back plywood pieces. With the aluminum foil, I glued it onto the plywood with spray adhesive, a trick I learned in a kid's workshop for making cardboard solar cookers. (We'll see how it holds up.)

You can also see that I stapled the bubble foil insulation onto the lower sides of the dryer. Below is the kitchen aluminum foil (on the left of the photo) glued to the front plywood section of the dryer. Just to the right of this is visible a thin section of the bubble insulation that is glued and stapled to the bottom.
Mr. Fodor recommends the foil insulation on the larger back door of the dryer as a performance upgrade to get more drying time later into the season. He suggested using foil tape. I've taken this idea a lot further. The question is will the dryer be too hot now in the summer? I'm starting to get a little nervous about this.

I built the dryer with his suggested electrical back-up of two standard 200 watt incandescent light bulbs. These are to help get drying done on cloudy days or later in the fall when there is less sun. The bulbs I found were 130 volt commercial-service bulbs. The box states that at 120V (more like regular house current) the energy used is 176 watts. Nevertheless when the dryer is plugged in and the vent doors are closed, the internal temperature pretty quickly reached 160 degrees! I suppose with the extra insulation I could use a lower wattage bulb. Is there such a thing as a 150 watt bulb?

A few general comments about the design:

One thing that made me nervous in the plans was the placement of the light bulbs in the bottom of the dryer. The 200 watt bulbs seemed too close to a piece of wood that supports the dryer's bottom section of plywood. Mr. Fodor's suggestion to limit the fire risk was to apply aluminum tape to the wood below the bulbs. My method was to wrap the support wood a few times in foil. (I didn't have any foil tape.) I stapled the foil onto the wood. I like having a few extra layers of foil to provide some air insulation between the foil and the wood.

To help move the the bulbous part of the bulbs away from this wood, I mounted the porcelain sockets on an additional section of plywood. This help give a bulbs another 1/4 inch of clearance from the wood. I mounted the sockets as close as possible to the metal collector plate so that the bulbs are almost touching. This might help dissapate some heat. Now that I see how well the extra insulation works, maybe I can get away with lower wattage bulbs? This would save electricity and add a margin of safety to the design.

The book suggests that the exterior of the dryer can either be painted or coated with linseed oil to provide some protection from getting wet in the rain. I coated the plywood ends and the exterior in shellac. I'm a fan of shellac because it is natural and easy to work with. The solvent/liquid medium for shellac is denatured alcohol, pretty benign stuff compared to other chemicals and solvents. I'm not sure how well the shellac will hold up outside in direct sunlight. I've read that one method of sealing shellac from UV degradation is to coat it with old-school (floor?) wax. I may wait to see how well the shellac holds up for a season before trying this. Alternatively, shellac is so easy to work with, maybe if is begins to fail I will just slop on another coat?

Now all I need is something to dry! Another challenge will be figuring out where to store the dryer. It's a bit big and our yard space is limited. If I can talk Becky into sewing a rain cover, it might work well on our flat garage roof. If that doesn't happen I'll hoist it into the attic.