N. Srisittipokakun et al. / Procedia Engineering 32 (2012) 839 – 846
1. Introduction
Thailand is an agricultural country, and its products range from world famous jasmine rice to various
vegetables, fruits and herbs. Most of the products need some kind of preservation to enhance their shelf
life since the production usually exceeds market demand at the harvest season. Drying is one of the most
used methods for product preservation, and as a result, it adds higher value to the products. A dryer can
achieve this purpose by supplying more heat which in turn increases the vapor pressure of the moisture in
the product, reduces relative humidity of the air, then increases its moisture loading capacity and ensures
sufficiently low equilibrium moisture content. Solar energy can be used as an important and
environmental compatible source of renewable energy. The use of solar energy for drying effectively
reduces the problems arising from generating energy by convention method. This is because the use of the
conventional energy source for drying purposes is costly and hazardous to environment. Solar drying
system may be classified into direct, indirect and mixed modes. In direct solar dryers, the air heater
contains the product and solar energy passes through a transparent cover and is absorbed by the product.
Essentially, the heat required for drying is provided by radiation to the upper layers and then conducted to
the product bed. In indirect drying system, solar energy is collected in separate equipment, called solar air
heater, and the heated air then passes through the product bed [1].
Andrographis paniculata is an annual plant with characteristic white-purple or spotted purple flowers
that flourishes in South-East Asia, China and India. It has been valued for centuries by herbalists as a
treatment for upper respiratory infections, fever, sore throat and herpes. Other reported applications
include its use in cases of malaria, dysentery and even snakebites. Now, important new research has
confirmed a host of pharmacological benefits for this herb, including potent anti-inflammatory, antibacterial
and anti-viral effects. In addition, scientists have discovered that Andrographis paniculata helps
boost the immune system, protects against cancer, prevents blood clots and maintains efficient digestive
functioning [2]. The purpose of this paper is to present the developments and potentials of solar drying
technologies for drying Andrographis paniculata.
1.1. Design of Solar Dryer
To carry out design calculation and size of the tunnel dryer.
The mass of water to be removed during drying, Mw kg [3];
mw = (Mi – Mf)/(100 – Mf) x mi (1)
where mw is mass of water to evaporation, (kg), mi is mass of the dried product, (kg), Mi is initial moisture
of product, (%, wb) and Mf is final moisture of product, (%, wb), then calculate the amount of heat
required from the equation [3];
Qdrying = mwL. (2)
when Qdrying is amount of heat required, (MJ) and L is heat required to evaporate water 1 kg, (MJ/kg). In
the final step, calculated area of collector from the equation [3];
A = Qdrying /(