Eben’s Solar Food Drying Blog — Spring 2012

Adding Electric Backup Heating to SunWorks™ SFD Kit

Here in Eugene, Oregon, I like to install backup electric heating in my SunWorks Solar Food Dryers. I still do more than 90% of my food drying with solar energy, but I really value having a backup if the weather changes, or if I need to dry at night or during the off-season.

Here’s a description of the simple, effective, and inexpensive system I added to my SunWorks SFD Kit dryer using light bulbs. This backup system works so well in my solar dryers that I haven’t used our electric dryer in years.

Why use bulbs for heating?

After researching various electric heating options, I decided that the simple approach described in my book (The Solar Food Dryer) is still the best. It uses two 200 Watt incandescent light bulbs to generate 400W of backup heating. It’s the cheapest possible backup system, costing less than $25 for all the parts. Incandescent bulbs generate about 95% heat and only 5% light, which is why they make a good heat source. Most of the 5% of light energy can also be captured and converted to heat in the dryer to achieve about 98% heating efficiency.

The one drawback is that the filaments inside most light bulbs are fragile and they will fail if the dryer is dropped or banged around a lot. Hammering on the dryer is one way I have found for shortening bulb life. Otherwise, the bulbs should get their 750 hours of rated operating life. Depending on how much the backup heat is used, 750 hours may last 10 years or more. At about $3.50 each, replacing these bulbs is fairly inexpensive. I remove the top glazing and reach under the absorber plate to switch them out.

What will I do when incandescent bulbs are phased out?

Incandescent bulbs are still readily available throughout the U.S. We are beginning to see modest efficiency improvements in the incandescent bulbs in the 40W to 100W range for general household lighting applications. However, specialty bulbs and utility bulbs (like the 200W bulb) are likely to continue to be available in their conventional form for many years.

If incandescent bulbs were to become completely unavailable, I would probably substitute two “cone heaters” of similar wattage. This type of heater is made to screw into a standard light fixture and has similar proportions to the standard light bulb. Cone heaters are much more expensive than incandescent bulbs, so bulbs are my first choice.

My safety protocol

I follow several steps to maintain safe operation of electric backup heating for my dryer. First, I always plug it into a GFI-protected electrical outlet. A GFI, or ground fault interrupter, detects any shorting or leakage of electricity in the circuit and shuts down the outlet. I make sure all my outdoor receptacles are on grounded circuits and have GFI outlets installed. GFI outlets are usually labeled and have a test/reset button right on the outlet face. They are available at any hardware store for about $12 and I consider them essential for use with all outdoor electric lighting, appliances, and power tools.

Second, I am always cautious when using an extension cord outdoors because they can easily become cracked and frayed. I plug the extension cord into the dryer first, and plug the other end of the extension cord into the GFI outlet last. When unplugging the dryer, I removed the cord from the outlet first before unplugging the dryer and winding up the cord.

The Parts

The parts for the backup heating system can be found at True Value and Ace Hardware Stores, and perhaps other local hardware stores. The parts list consists of the following:

Parts List:

  • 2 compact, surface-mounted, porcelain light fixtures
  • 2 - 200 Watt incandescent, general purpose light bulbs
  • 3 feet of 16-gauge lamp cord and plug (cut from 6-ft. black or brown extension cord)
  • 4 - 1-inch-long, #8 pan-head metal screws for mounting fixtures
  • Small package of 16-guage, spade type, wire terminals (6 needed)

Notes on parts: The porcelain fixtures shown in the illustration have a small 2-1/4” diameter base, which fits nicely on the rear panel of the SunWorks dryer. I found it difficult to securely attach the cord wire directly to the terminals in the porcelain fixture, so I purchased some spade-type wire terminals and attached them to the wires before connecting them to the fixture. The 6 ft. extension cord is cheaper and easier than buying the wire and a separate plug.


My SunWorks SFD Kit unit was already assembled. To get access to the inside of the rear panel (where I wanted to mount the light fixtures) I removed the food trays and the thermometer from the dryer and turned the dryer upside down on a large sheet of cardboard (to protect the glazing and trim). Next, I removed the bottom panel by removing all the screws.

With the dryer upside down and the bottom panel removed, I marked the rear panel 6” in from each leg (7-1/2” from the inside of the side panel), as shown below. (The rear panel is the panel below the access door.) These marks served as the centerlines for locating the fixtures horizontally. Each fixture was centered vertically between the two wood support rails on the rear panel.

Once the location of the fixture was identified, the two mounting holes on the fixture were marked on the rear panel using the point of an awl. The screw holes were drilled ¼” deep using a 3/32nd drill bit. I selected 1-inch-long mounting screws for this particular fixture to assure a secure attachment. I attached both fixtures and was careful to get the mounting screws snug, but not to overtighten them, to avoid cracking the porcelain fixture.

A ¼” diameter hole was drilled in the lower rear corner of the left side panel for the electrical cord. (Note: “left side” refers to left when dryer is upright and the front is the side that faces the sun. It is the side that doesn’t have the thermometer). The outlet portion of the extension cord was cut off and discarded. The bare cord end was fed through the hole in the side panel from the outside. A single knot was tied in the cord on the inside of the dryer about 8” from the plug end to keep the cord from pulling out of the dryer. This knot sets the height that the plug rests off the ground. I wanted to keep the length of cord outside the dryer short so that the plug would be about 4” off the ground. This is so any future connection with an extension cord will not rest on the ground where it could possibly end up in a puddle of rainwater.

After carefully determining the length of the cord needed to reach the terminals of the first fixture (taking into account the wire terminals), I cut the cord. I split the end of the cord and stripped about ¼” of insulation from the end of each wire. I added wire terminals by crimping them firmly onto the bare wire ends with pliers.

The remaining cord was used to cut a section to connect the second fixture in parallel with the first fixture (see wiring diagram). Wire terminals were added at both ends of this section of cord and the wiring was put in place to check for fit. Once the fit was determined to be okay, the fixture terminals were tightened and the wiring was complete. I screwed in the two bulbs and plugged in the dryer to check the bulb operation. After installing the bulbs I was careful not to knock or bang the dryer to avoid damaging the filaments in the bulbs.

I added reflective aluminum tape to the area of the bottom panel under the light bulb (top side only). This reflects some light back up to the absorber plate and helps insulate the bottom panel from the heat of the bulbs. The aluminum tape was left over from when I assembled the unit and added reflective tape to the inside of the access door.

I find that light bulb wattage is fairly accurate, and that 400 Watts provides a nice level of backup heating that raises the dryer temperature about 70 degrees above ambient (outdoor) temperature when the adjustable vent is closed. This will produce steady drying when the ambient temperature is at least 45°F.

Now my SunWorks solar food dryer is ready to be a workhorse in the garden and I can enjoy worry-free food drying any time of year!

Happy solar drying!


Copyright ©2012 by Eben Fodor


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