Last edited: 10 November 2019      
SunPortal upper reflector

The SunPortal with upper reflector

Luther Krueger from Minneapolis is an avid collector and user of solar cookers. One type of cooker that appealed to him was through-the-wall solar oven designed by the late Barbara Kerr. Krueger thought this could be a great built-in appliance for any home with one exterior wall facing the sun--or a roof-top cooker with access from the inside of just about any house or top-floor apartment.


The SunPortal with lower reflector

Starting with Kerr's design, Krueger imagined a cooker on the south wall of his garage in Minneapolis (45 degrees latitude north). The first problem to solve was the obstacle a cooker jutting out from the wall posed for parking in the driveway by the garage. A cooker at "microwave" or eye level could risk a collision with his family's cars. The solution was to embed the cooker in the wall so that the glass was flush with the siding, looking like a regular window.


The SunPortal with both upper and lower reflectors

Krueger built a cook box with a window frame. The cook box is lined with 1" polyisocyanurate insulation—closed cell foam that is used in several manufactured box cookers. The window is double-glazed/thermal with polycarbonate "Lexan", and sealed with silicone. The box is roughly a 50 cm (19.7 in) cube inside the garage, with the back door of the oven hinged with a latch to cinch it tight when cooking. A barbecue thermometer is inserted into the plywood and through the insulation, toward the bottom of the cooking area, to indicate the lowest temperature achieved.


Inside view of the SunPortal

This first improvised effort netted around 99 °C (210 °F). Krueger decided to experiment with reflectors to increase the temperature. Two wardrobe glass mirrors hang from a hinge below the "window", about 55 cm (21.7 in) by 115 cm (45.3 in). The reflector is raised on a folding table "leg" so that it will lay flat against the garage wall when not in use, and flap out up to 90 degrees to send more sun into the box. When installed, this boosted the temperature to a more respectable 125 °C (257 °F) degrees. The mirror is set to capture sun early in the day, without adjustment through the afternoon, since the 115 cm (45.3 in) is parallel to the ground.

The glass mirror reflected a lot of light into the cook box. But Krueger wanted to push the performance higher, and added a second reflector overhead, made from spectral aluminum sheets. Two large sheet are glued to a thin plywood flap about twice the size of the lower reflector. The upper reflector is hinged at the top of the cook box window frame, and set in place with a pulley. The rope is tied to a cleat when the optimum amount of sun reflects across the glass. This bumped up the temperature to 160 °C (320 °F) during the summer months.

Krueger hopes that this cooker design would be attractive to window manufacturers, and green-builders/contractors could adopt a variation on the design as a selling point for off-the-grid and environmentally conscience home builders. It could also be a standard feature in a tiny house, with the cook box being part of a pantry cabinet. Ideally a crank mechanism from the inside would adjust the reflectors, so the cook would never need to leave the house.

Krueger has cooked successfully many times with this cooker and considers it a "proof-of-concept" design that could stand much revision, and welcomes others in the solar cooking community to run with it.

See alsoEdit


Luther Krueger
Minneapolis, Minnesota


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