# Thermal Evaluation of a Hybrid Dryer with Solar and Geothermal Energy for Agroindustry Application
## 1 Introduction  
Dehydration of a food consists of the extraction of water from the product with the aim to preserve it for much longer [1]. To achieve this process, drying equipment that requires a constant source of heat such as electrical resistors, biomass boilers, gas burners, and diesel burners, among other options, are used within the context of the industrialized framework. In the same vein, the small producers harvesting between 90 and 909 kg of cereals use wood, LPG (liquefied petroleum gas), coal, solar energy, among others, as an energy source for drying. It should be noted that the use of the selected energy source depends on the temperature, speed, and drying time of the product as well as the type of product to be dehydrated to avoid damaging its molecular structure and therefore its nutritional quality. It is estimated that the energy required to dry grains, depending on the drying temperature, ranges from 3000 to 8000 kJ/kg of water removed [2]. Likewise, the type of drying system as well as the quality and humidity of the grain can considerably affect the energy demand of the process. In relation to the evaluation of the energy efficiency of the process, several authors have reported in general terms that the consumption heat to dry grains varies between 2330 to 2790 kJ/kg of water eliminated when natural air is used, while on the other hand, when an artificial source of air is used (turbulent flow), this rate varies between 2790 to 3490 kJ/kg of evaporated water. Finally, for Batch-type dryers, the evaporation rate is usually 3490 to 4650 kJ/kg of evaporated water, and for high-temperature dryers without recirculation, the rate is 4650 to 6980 kJ/kg of evaporated water [3]. These values can present important fluctuations in the drying equipment, depending on the technological parameters and operation including energy vectors, which causes a significant increase in the final energy consumption [2,3]. In 1996, the Food and Agriculture Organization (FAO published in the booklet9 \"Post-harvest Technology\" in the document entitled \"drying of grains and dryers\", the basic procedures for the evaluation of drying behavior at the industrial level. This document highlights that there are two procedures. The first procedure consists of the realization of technical tests under norms or regulations previously recognized by accredited institutions with a high cost of execution. The second procedure, of lower cost, is based on gathering information from the data provided by the manufacturers themselves in their manuals or brochures or those that are acquired through direct contact with the owners of the dryers. In the case of Ecuador, the existing information in relation to the energy evaluation of the drying technology associated with the design and construction of prototypes based on the use of renewable energy sources for cereal drying is minimal. Likewise, it is necessary to indicate that many dryers in the agroindustry sector in Ecuador are of the artisanal or semi-artisanal type. These dryers have problems due to not controlling the temperature and drying speed by the hot gas in the chamber. This causes inhomogeneous drying, and the extra consumption of fuel unnecessary for its operation. Another important point of the current problems of drying in Ecuador is the need to build dryers with national materials to avoid the importation of Brazilian, German, Indian, Colombian and Chinese dryers, which have an approximate cost $7700 dollars (the reference value includes the import cost) for a capacity of 909 kg of product. In any case, local construction will reduce the cost of the dryer by about 50%. It could even regulate different drying temperatures (40 \({}^{\circ}\)C, 50 \({}^{\circ}\)C, and 60 \({}^{\circ}\)C) with the purpose that it can be used to dry any type of product (rice, wheat, corn, cocoa) [4]. Finally, during the research phase, it was determined that a handcrafted dryer built with materials such as cement and brick would cost $500 dollars for a capacity between 227 to 454 kg/day of product.