Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 140

or using the nomogram, the value of m J can be read off from the second quad-
rant (moving anticlockwise). Stage three is using the characteristic curve, X from
which the solar 10.71. This calculation corresponds to the third and fourth quad-

rants of the nomogram. The total thermal load, Q tot , when M L kg of water are heated
from the mains cold water supply temperature, T m to the required temperature of T L is

=

10 6 J

Q tot = M L C w ( T L − T m ) = 27 . 02 × (4.4.22)

From the definition of the Brooks number, X the daily solar fraction can be determined:

XM L C w ( T a − T m )

f = = 0 . 345 (4.4.23)

Q tot

4.5 SOLAR ENERGY COLLECTION AND STORAGE
FOR DRYING CROPS

High crop losses can ensue from inadequate drying, fungial attacks, and rodent and
insect encroachment in traditional “open-sun'' drying. Solar-energy tropical-crop dry-
ers that enclose the crop and enable air to circulate around it compete economically
with traditional open-sun drying because they

(i) require a smaller area of land in order to dry similar amounts of crop that would

have been dried traditionally in the open,

(ii) will yield a relatively high quality of dry crop, because insects and rodents are

unlikely to infest it during the drying process,

(iii) have a shorter drying period,

(iv) they afford protection from sudden rain,

(v) incur relatively low capital and running costs, and

(vi) give improved crop quality achieved after drying.

In an integral type natural-circulation solar-energy dryer, the crop to be dried is
placed in a drying chamber with transparent walls; heat is supplied to the crop by direct
absorption of solar radiation and by convection from the heated internal surfaces of
the chamber. The heat abstracts the moisture from the product, while also lowering
the relative humidity of the resident air mass thus increasing its moisture carrying
capability. The direct absorption of solar radiation makes these dryer particularly
appropriate for greenish fruits as during dehydration, the decomposition of residual
chlorophyll enhances the proper colour “ripening''. For certain varieties of grapes and
dates this direct exposure to sunlight is considered essential for the development of
the required colour in the dried products. Similarly exposure to sunlight of arabica
coffee develops full flavour of the bean. Conversely, for some fruits, exposure to sun
reduces considerably the vitamin content or blemishes pigments; such produces are
thus best dried in enclosed opaque-wlled cabinets or silos to which solar heated air is
provided.