As it is usual in solar applications, the low radiation density and the variability
of the energy source are the main problems to solve. It is quite simple to add an
auxiliary heater to this drier. Fig. 10 shows the operation of this system either in
cloudy days or at night. Its size is selected to produce the drying gradient and the
daily production obtained during sunny days. The collector ambient air input is
closed improving the thermal insulation, since it produces an air chamber with low
natural convection surrounding the drying tunnel. The ambient air is taken directly
from the outside South side, and it is pushed by the fans through an unfinned tubular
heat exchanger, where it is indirectly heated by the combustion gas flowing inside
the tubes. Then, the heated air moves through the file carts as before. The hot gas
is produced by a forced flow stove, fed with firewood or agricola surplus. The
remaining cooler combustion gas is expelled to the environment by a chimney. Both
the heat exchanger and the stove are insulated to avoid heat losses and to protect
the plastic cover.