Solar Cookers and Food Dryers by Cookbooks

Author:Cookbooks [Cookbooks]
Language: eng
Format: epub, pdf
Publisher: IB Dave's Library
Published: 2010-05-10T14:54:26+00:00

could be closed or removed when transporting the dryer

routinely attain temperatures of 130–180° F (54–82° C)

(see Photo 5). Graph 3 shows the significant increase

without reflectors, which is hot enough for food drying

in temperatures attained by using these reflectors. The

and for pasteurization. Based on our work so far,

problem with this design was that if the dryer could not reflectors just don’t seem to be worth the trouble.

track the sun for one reason or another, one of the

Absorbers

All low temperature solar thermal collectors need

Figure 6: Ideal Angle for Side-Mounted Reflectors

something to absorb solar radiation and convert it to

heat. The ideal absorber is made of a conductive

material, such as copper or aluminum. It is usually thin, without a lot of mass, and painted a dark color, usually black. The original dryer design called for five layers of Graph 3: Vertical Wall & Side Reflectors

vs. No Reflector

180

160

140

Reflector

Reflector

120

Surface

Surface

100

Collector Surface

Degrees F

80

60

60°

Reflector

Angle

40

20

8:00

9:00

10:00

11:00

Noon

13:00

14:00

15:00

16:00

Time

With Reflectors

Without Reflectors

Ambient

Home Power #69 • February / March 1999

29

Solar Dehydration

Figure 7: Collector/Absorber

Configurations

Normal Diagonal Absorber

Reverse Diagonal Absorber

Steeply Angled Sections

Dual Pass

U-Tube Through Pass

black aluminum window screening, which had proven to

Lath vs. Screen

work well in other air heating collectors we had

Next, we compared three layers of pre-painted black

constructed. Other designs call for metal lath, metal

aluminum window screening to three layers of

plates such as black metal roofing, or aluminum or

galvanized steel lath painted flat black. We found that

copper flashing. We decided to try some different

the lath produced temperatures as much as 15° F (3.6°

materials and approaches to see if we could come up

C) higher than the screen in our outdoor solar food

with a better absorber.

dryer tests. We got the same results when we

compared six layers of screen to six layers of lath (see Plate vs. Screen

Graph 4). While we found that the lath produced slightly First, we compared five layers of black aluminum

higher temperatures, it was harder to work with, needed

window screen placed diagonally in the air flow channel

to be painted, and cost slightly more than the screen.

to one piece of black corrugated steel roofing placed in the middle of the channel (see Figure 7). We found that

When these tests were replicated with the solar

the mesh produced temperatures about 7° F (3.9° C)

simulator, we had slightly better results with the screen higher than the roofing in full sun. Other experiments

than with the lath in both the three and six layer tests.

have shown that mesh type absorbers are superior to

We were disappointed by the lack of positive correlation plate type absorbers. These differences might be

between our outdoor tests with the actual food dryers

reduced if we used a copper or aluminum plate instead

and our indoor tests with the solar simulator. But there of the steel roofing.

are many variables to control and quite a few people

involved in setting things up and collecting data, so our Graph 4: Lath vs. Screen Absorber

control was not as tight as we would have liked. Despite 180

these problems, we are confident in concluding that

there is not a great deal of difference in performance

160

between lath and screen—both work effectively.

140

Layers of Absorber Mesh

We then compared three layers of lath to six layers of

120

lath, and three layers of screen to six layers of screen.

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