2. Solar dryers with latent heat storage materials: a review
Very limited information is available regarding the use of latent
heat storage to conserve thermal energy during drying. Devahastin
et al. [37] proposed, via numerical simulation, the use of latent
heat storage to store energy from the exhausted gas of a modified
spouted bed grain dryer. A saving of up to 15% could be achieved
with the use of such combination. Devahastin and Pitaksuriyarat
[38] investigated the feasibility of using latent heat storage (LHS)
with paraffin wax (Fig. 2(a and b)) as a phase change material
(PCM) to conserve excess solar energy during drying and release it
when the energy availability is inadequate or not available and its
effect on drying kinetics of a food product (sweet potato). Heat
transfer characteristics, temperature profiles as well as the effects
of the inlet air temperature and velocity on the charge and
discharge periods were investigated. It was found that melting was
dominated by heat conduction followed by free convection;
melting took place from the center of the LHS to a point far away
in the radial direction and took place from top to bottom points in
the axial direction. However, only heat conduction was dominant
in the solidification process. PCM froze from an outer to an inner of
the LHS tank due to heat loss to the surrounding. Charge time
decreased with an increase of the inlet air temperature and air
velocity. The amount of extractable energy per unit mass flow rate
of inlet ambient air was 1920 and 1386 kJ min kg1 when using
inlet air velocity of 1 and 2 ms1, respectively. This LHS could save
thermal energy during drying of sweet potato by approximately
40% and 34% when using inlet air velocity of 1 and 2 ms1,
respectively.
Recently, Bal et al. [39] designed and developed a solar dryer
with a latent heat storage (LHS) with paraffin wax as a phase
change material (PCM) to store excess solar energy during the day
time (by using hot air at temperatures close to those exhausted
from a typical solar collector) and release it when the solar energy
availability is inadequate or not available (by forcing ambient air
through the energy storage to extract the stored energy), which
implies a possibility of reducing the amount of supplementary
energy required in the drying operation and after all continuous
drying of agricultural/food products at steady and moderate
temperature of 40–75 8C can be possible. Half split bamboo was
used in solar dryer to reduce cost as it has good thermal insulation
compared to metal and reasonable mechanical strength. Initial
measurements of temperature at different points such as inlet,
outlet, on black panel and below panel of solar panel with free
natural circulation of air and water have been carried out daily. The
desired outlet temperature has been achieved for drying of food
materials. One main drawback of heat loss in the initial model was
rectified by adding a coating of PU foam below the panel. The
drying system is shown in Fig