Drying of food
Practical Action
Is the site appropriate for solar drying?
To decide whether solar drying is a realistic option, you need reliable data on sunshine levels, cloud
cover, rainfall and relative humidity. In some places this data is available from a local institute or a
government department. If it is not readily available, you can use a scoring questionnaire on the local
climate to help you make the decision. This system was developed by the Brace Institute (taken From
Axtell, 2002).
Question
1. Do you know what the average daily radiation on
a horizontal surface is, in the area where the dryer
is to be set up and for the drying period (in
kWh/day/m2)?
Options
Less than2, score 0
2-4, score 3
4-6, score 6
More than 6, score 9
Score
If you do not know then answer questions 2, 3 and
4 to try and estimate it. Otherwise go to question 5
taking this value as TOTAL (1).
2. What is the average number of hours of
sunshine per day?
3. What is the length of the day in hours from
sunrise to sunset?
4. What kind of clouds are there?
ADD TOGETHER THE SCORED FROM
QUESTIONS 2, 3 AND 4 TO GIVE TOTAL (1)
5. What is the average number of consecutive days
on which the sun shines for less than four hours?
6. At what latitude is the dryer sited?
7. At what altitude (in metres) is the dryer sited?
8. How do you describe the climate?
9. What is the average air relative humidity during
the drying period?
(Air with a humidity above 85% is uncomfortable
and objects stick to the skin. In a climate with less
than 45% humidity, clothes dry out very quickly).
10. Are there particles in the air from dust, sand,
car exhausts or smoke?
ADD UP THE SCORES OF QUESTIONS 6, 7, 8, 9
AND 10 TO GIVE TOTAL (2)
Calculate: TOTAL (1) + TOTAL (2) -5 = TOTAL (3)
14
Less than 4, score 0
4-7, score 1
7-10, score 2
More than 10, score 3
Less than8, score 0
8-10, score 1
10-12, score 2
More than 12, score 3
Thick score 0
Thin and misty score 2
No clouds, score 3
If TOTAL (1) is:
< 3, availability is 2kW/h
3-6, availability is 4hW/h
6-8, availability is 6kW/h
>8, availability is 8kW/h
None, score 0
One, score 1
Two, score 2
Three, score 3
Four, score 4
More than 45°, score 0
Between 30-45°, score 1
Between 15-30°, score 2
Less than 15°, score 3
Less than 500, score 0
500-1000, score 1
1000-2000, score 2
2000-3000, score 3
Above 3000, score 4
Hot and humid with little variation in
temperature, score 0
Mild, temperate and maritime, score 1
Continental with large variations, score 2
Desert or semi-arid, score
Above 85% (very humid), score 0
65-85% (quite humid), score 1
45-65% *quite dry), score 2
Less than 45% (very dry), score 3
High concentration, score 0
Low concentration, score 1
None, score 2
If TOTAL (3) is :
Less than 6. The physical conditions are
not right for solar drying. You will need
other systems to provide energy.
Between 6 and 20. Another source of
energy will be required as a stand-by.
More than 20. Conditions are in favour of
using solar energy.