Design of a Combined Solar Dryer: Thermal-Photovoltaic for the Treatment of Grains

Solar drying is one of the most vastly used technologies in rural areas, especially in developing countries, allowing for self provision and commercialization of products with an added value. Among the technical problems encountered, is the control of the operating conditions, due mainly to the intermittency of solar radiation which in turn produces a non homogeneous treatment of the product making it necessary to control both temperature and air velocity. Considering these facts, the design of a dryer for the treatment of grains is proposed, with the intention of obtaining control on the kinetics of drying using direct or indirect solar heating and electrical energy storage from a solar photovoltaic system within established air velocities and temperatures ranges. The energy required for the grain treatment is provided by a solar thermal-photovoltaic multipurpose solar unit installed in the solar testing platform at CIE-UNAM. Solar heating can be carried out by indirect heating of air using a thermo hydronic system with a water to air heat exchanger or by means of an one pass solar air heater. The heating of air can be direct during the day or using thermal storage both at night as well as the following day. The dryer can be operated during the day or the night or 24 hours a day if it is required, depending on the thermal and photovoltaic system configurations. The photovoltaic system is composed of photovoltaic panels, batteries and a charge controller and it has been designed to operate the fan motor as well as the lamps in the night time, however a total solar operation is possible by activating circulation pumps between the solar array and the dryer, between the solar collectors field and the thermal storage tank and between the storage tank and the dryer. Part of the electricity generated is for day use for the fan and the other electric components and the rest is stored for night utilization. This paper presents the conditions and parameters for the design of the grain dryer, the different heating circuits and the integration aspects of measurement and control devices of the multipurpose solar unit, as well as the operating modes.