Wednesday, April 1, 2015

How (and How Not) to Make a Do-It-Yourself Solar Food Dryer from an Old PC

Picture of the complete solar passive / active food dryer
The complete DIY solar food dryer. A passive dryer is underneath the solar panel.
Source: All photos by Casey Bahr

Solar Food Dryer Designs

A "quick and dirty" way to make a solar food dryer is to build a screen tray into a box and top the box with a piece of metal roofing painted black. Tilt the box slightly to increase airflow and you're done. Another design often seen is a set of shelves for the food racks with the frame covered by clear panels, such as polycarbonate roofing, which is resistant to UV damage.
The project detailed here takes an unconventional (some might say geeky!) approach. Though the first prototype was a failure, a quick modification created a working food dryer. This dryer is both solar active and passive. That is, it generates heat passively, but uses solar generated electricity to circulate the hot air, which improves its efficiency markedly. 

It's also a great way to recycle any old PC cases, which are easy to locate. Also, note that the food itself is not exposed directly to any damaging sun rays.
Old PC case to be gutted.

Old PC with components to be stripped out
Original PC case

Creating the Dryer Box

This project is one of those using materials on-hand, which is one of the author's favorite methods for do-it-yourself inventions. An old desktop PC case holds the drying racks. The natural heat-collecting property of the black metal case and a fan to blow air across the racks from the outside was the plan.

Clean pc case after being stripped down
PC case stripped down

The first step was to strip out any unnecessary components from the inside of the case, so that you have an empty metal box. If there is still a case fan or power supply fan in the "guts", save that aside. It should be marked as running on 12 volts, which most PC fans do.

A door was cut on one end into which the drying racks will slide.Great time to get out your reciprocating saw. Attach the hinges first to the top and mark the hinge holes on the door panel before cutting

Door and Rack Supports

This particular PC case is a clam-shell variety, which gives solid walls on three sides. Using a small handheld grinder with a cutting disc, a door was cut on one end into which the racks slide. Before cutting the side door, the hinges were screwed onto the top panel with self-tapping truss head screws

Fold the hinges down over the door side, and mark the panel through the hinge holes. This step will ensure good alignment of the door. Cut the top of the side panel where the hinges are and then re-attach the panel via the hinges with the same kind of truss head screws.
Rack supports are made from pieces of 1" by 2" wood; held in place with screws through the flanges on the bottom part of the case. If there are a large number of holes in the bottom or side panels, cover them with thin foam, plastic sheets, or even cardboard. You don't want it to be airtight, however, since air needs to escape to keep an even air flow.

Air distribution tube, later abandoned

Now for the Ugly Part

Improvisation is a valuable virtue when approaching any DIY project such as this one. In this case, an adapter was needed from the fan to the smaller hole for the air spreader. 

Using three different sizes of PVC tubing and a heat gun, a solution of sorts was found. The first piece of tubing, which entered the rack case, was 1-1/4" electrical conduit, connected to a 2" piece of PVC waste pipe, which in turn connected to a piece of 3" PVC pipe to match the fan diameter.

This tube from the failed prototype lay on the bottom of the case. The numerous holes help spread the air from the fan evenly. An adapter was made for attaching the PC fan to the distribution tube by using a heat gun to melt a 3" PVC scrap tube.

Notches were cut into the large end of this home-brewed adapter. These tabs were heated just enough so that they could be easily bent downwards to form mounting tabs to which the fan was attached with screws.

heat gun melted pipe adapters
Adapters from the fan end to the distribution pipe

To create the intermediary adapters, heat the end of the tubing with a heat gun and wearing heavy gloves twist the end of the pipe into a smaller diameter. The smallest tube was inserted through the PC case along one side. It had two rows of 1/4" holes drilled along its length to spread the air out from the fan. 

If at First You Don't Succeed

Unfortunately, as sometimes happens with DIY projects, the initial design failed.
Heat collector with fan covered with glass. The envelope insulates a small remote thermometer

An Improved Design with Heat Collector

The first prototype of the solar food dryer simply didn't generate enough heat. Though the drying box would get warm, all the heat was lost as the fan pulled in cool air from the outside. This deficiency was overcome by creating an external heat collector using a second PC case. 

A tower case, which by chance had a fan already mounted dead-center on the back panel, was employed. The ugly plastic tubing was happily discarded.
Screw the heat collector directly to the drying rack case after cutting a hole (with a jigsaw) in the drying case the same size as the fan. Conveniently, a tower case is enclosed already on five sides with a removable panel on the sixth side. This removable panel was set aside and replaced with a piece of glass slightly larger.
To boost the heat generating capabilities of collector, it was lined on the bottom and sides with eighth-inch foam with a foil backing on one side. A piece of corrugated roofing was screwed into the bottom for additional heat mass and everything painted flat black.

By Golly, It Works!

Hook-up and Testing

A small wireless temperature sensor placed in the heat collector registered about 140 degrees Fahrenheit within about 10 or 15 minutes. When the fan was hooked up, the temperature dropped to about 112 degree F. and remained steady. That's with an ambient temperature of about 78 degrees F.
So much for the passive part of the design. The fan is run off a 5 watt solar panel, the kind used for topping off car batteries. A capacity of half that size would be sufficient. The solar panel is connected to a solar charger, which connects to a small sealed lead-acid battery. A smaller battery would work just fine. The battery smooths out the supply of electricity to the fan if a cloud passes over.
The first food to be dehydrated was tomatoes. The two racks hold about four or five thinly sliced tomatoes and within 10 hours they are dry enough to be stored in plastic bags from which tomato paste was made later. 

That's about the same time it would take in our plug-in food dryer, but we saved about 4 kilowatt hours of electricity using the solar food dryer. So, that's success! Of course, there could be further improvements (add your ideas in the comments!), but throughout the summer this solar powered food dryer should be just fine for household needs.


  1. My PC sucks! It might be a food dehydrator yet. That is a good idea for up-cycling project. You could dehydrate food before it goes bad.

    That alone would save you money and you will always be prepared for a hurricane.


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